I agree with many elements of the previous answers -- it could be due to the wet dough "resealing" and/or to the crust hardening too early and preventing further expansion. Doing a more horizontal slash than a vertical one is helpful to get good "ears," and extra moisture will keep the crust softer for a little longer to get more oven spring.
Frankly, although I did it for years, I don't find the plant sprayer method to do very much -- and if you're opening the oven periodically to spray in the first few minutes, you could be losing significant heat that could actually reduce oven spring. I concur with the steam pan method or using an enclosed pot. Note that pots don't have to be cast iron: any enclosed pot will significantly improve your crust as long as it is oven-safe at the baking temperature.
That said, I think this particular problem is difficult to troubleshoot without observing your specific loaves and slashing technique. Slashing deeper (whether vertically or horizontally) is definitely NOT always the answer and can actually deflate your loaves significantly if done incorrectly. With proper hydration, shaping, and oven steaming, it's very possible for quite shallow slashes to lead to great expansion. (As an aside, serrated knives are also about preference -- if you don't keep your straight-edge knives very sharp, serrated may be a better choice. However, they can leave your slashes jagged on the final loaf, which may not be as pretty and may result in unevenness for shallower cuts. I keep some of my straight-edge kitchen knives really sharp so they can be used for things like this.)
A lot of it depends on shaping and the stage of proofing your dough is in when you bake it, as well as the hydration. If you do a very thorough shaping (i.e., preshaping, bench rest, then very tight shaping of the final loaves), the outermost skin of the loaf may be very taut. Even a shallow slash could be enough to allow the loaves to open up significantly. If, on the other hand, you do a very gently shaping (little or no pre-shaping, trying very hard not to deflate the dough at all), the "skin" will not have the same characteristics and deeper slashes may be necessary. (As an example of this, you might consult the different advice given by Peter Reinhard and Jeffrey Hamelman -- the latter emphasizes detailed and tight shaping and thus advocates very shallow slashing; the former encourages gentle handling when shaping and advocates somewhat deeper cuts.)
The stage of proofing is also critical here: a loaf that is somewhat underproofed will likely have a more taut skin but will also hold its shape better even with deeper slashes. If the loaves are somewhat overproofed, they have a better chance of deflating or at least losing height with deeper slashing. The apparent moisture of the dough and its susceptibility to "reseal" the cut will also change depending on proofing stage.
Again, there are a lot of factors to consider. I would pay particular attention to the behavior of the loaves right after you slash them. Do the slashes spread significantly immediately (indicating a taut surface)? Or do they just remain close together (and thus risk re-sealing)? If you slash more deeply, does the loaf deflate? And if it does so, does it seem to reinflate in the oven, or do your permanently lose height? These observations can help troubleshoot the exact issue. A final concern -- sometimes if the dough is likely to reseal, waiting too long between slashing and getting the loaf into the oven can be a problem. A few seconds should not be an issue, but if you're taking a few minutes to slash and load a few loaves before getting them to the oven, that can be enough time for some cuts to close up again.
The theory is unlikely. The interior of dough rarely gets more than a few degrees above boiling, and it usually "stalls" for a significant amount of time in the 210-212F range. The only way to go above that is to dry the dough out completely, resulting in a cracker-like consistency.
That's the reason why the crust has a different texture, color, etc. than the interior. The crust is the portion of the bread which actually reaches temperatures significantly higher than boiling. That allows other types of chemical (and thus flavor) reactions to happen, like the Maillard reaction and caramelization.
Another way to think about this is that 212F is boiling at sea level, and bread dough contains a lot of water, and the large interior "surface area" surrounding each bubble means there is a huge amount of surface area for water to evaporate. So, if you heat water to 212F, it will evaporate, and it will leave all of those interior bubble surfaces at 212F. Until the dough dries out enough to slow that moisture production significantly, the interior will maintain a temperature of roughly 212F.
Furthermore, even if you somehow did create pressure inside the dough to raise the temperature inside significantly above boiling, it would exert significant pressure on every surface of the loaf, and it would burst at any location. Achieving even a few extra degrees of temperature would require significant extra pressure to be continuously exerted on the entire exterior of the loaf. So, "deep frying the crumb" would be really difficult to achieve, and would you want that? (Think of a hushpuppy or something that's deep fried dough -- would you really want the interior to be all crustlike and dried out?)
Regarding the flip -- I've never done it, but I assume it must just be for the shape and perhaps the coloring/crust consistency. Dimpling on focaccia is done for similar reasons: on thin dough, if you have giant holes, it can create areas that might expand and become misshapen or burnt or mess up the consistency. I've never dimpled to deliberately deflate the dough or decrease oven spring. I do it to ensure a relatively consistent loaf during baking.
Focaccia can have enough oil on the surface so that it effectively does "fry" a bit, and if you have excess oil in the pan and flip it partway through the bake, you will effectively panfry the crust. If your pan isn't that oily, I'm guessing it's only about the shape. (I also don't know why you'd like to limit focaccia's oven spring -- I've gotten the most compliments on mine when I've accidentally let it rise a little too far, resulting in something that was thicker than typical but a truly amazing tender interior.)
But the interior? You can only get the "fried" texture and flavor by raising the temperature significantly above boiling and getting rid of most of the moisture. But then you end up with an interior that tastes like a cracker (i.e., all crust). If you don't get rid of the moisture but raise the pressure, you end up with something that is sticky and bagel-like, not typical desired focaccia characteristics. (Don't believe me? Try baking bread in a pressure cooker -- which requires excess steam -- and see what happens.)
Best Answer
It looks like there may be some misunderstanding about how the cloche is supposed to work, and what scoring will/should look like. I think the bread in your photo looked like it didn't need any further scoring. I think it looked like a great result, since there aren't any visible blow outs on the bottom. If you preheat the cloche in the oven, and put the bread in, with one or two deep scores, you should get some spring and steam from the oven as the water escapes from the bread inside the cloche. This is what causes the wide opening of your score. The fact that it doesn't look like the "leaf" or "lip" that professional bakers get is mostly due to practice with the angle/depth of the score, and likely the amount of steam that is in the oven. The cloche will create some steam, but not as much as is in a commercial oven with steam jets. I don't see anything wrong with your pictured loaf. It may be different from what you were expecting, but I think it is perfectly fine. There was probably no need to do the second scoring.