Yogurt and milk might work, but those ingredients are not in authentic chapati. When I hear the words "hard" and "rubbery" in reference to pan-fried bread, I'm almost certain that the problem is that they are cooking for too long on too low of heat. Increase the heat on your pan (it should be very hot) and cook the chapati very briefly on each side. This will still give them a nicely browned exterior, without turning them into sheets of rubber.
The most recent Cook's Illustrated has a cinnamon swirl bread article that discusses this problem. According to them, the root cause is a lack of binding between the dough and filling. Gas from the yeast, and steam generated during baking, push into the spiral, creating pressure that compresses the dough and widens the gap.
In the specific case of cinnamon bread, they recommend using powdered (confectioner's) sugar instead of granulated, and using a large amount of cinnamon. The finer sugar dissolves more easily in moisture from the bread, quickly creating a paste that is reinforced by the cornstarch and by starches in the ground cinnamon. Misting the bread before adding the filling also helps.
In order to make a sausage filling adhere to the bread, you would need a sticky, water-soluble element. Cornstarch or powdered gelatin spring to mind immediately as possibilities.
The other tactic they employ is to actually expose the filling during proofing, preventing the yeast's gas (and later, steam) from building up alongside the filling. Once the filling is rolled into the dough, the loaf is cut in half lengthwise. With the two halves laid side-by-side, cut face up (exposing the filling), they are wound around each other, folding the left piece over the right until the end is reached (this is called a "Russian braid"). The ends are pinched together, and the loaf is placed in the pan to proof. This, obviously, should help with any kind of filling, not just cinnamon sugar.
Best Answer
I'll try to be specific without getting into too much detail here:
The chewiness (AKA elasticity) - of dough is due to the formation of gluten. This is affected by:
The crust (I assume this is what you're referring to by "softness") - is primarily the result of the Maillard reaction, which requires an amino acid and a sugar as well as heat. The longer and more quickly the reaction carries on, the crispier and browner the crust will be. The factors are:
Finally, the moistness of the bread is essentially a combination of the first two:
Higher fat content means less gluten is formed and more of the moisture is preserved, as long as it doesn't evaporate. The fat itself also adds a certain amount of moisture as far as mouth-feel is concerned.
Higher sugar content also preserves more of the moisture and using a wetter sugar (i.e. brown or muscovado) provides some moisture of its own - although the latter can easily evaporate with over-baking.
Longer baking times cause more of the water to evaporate, which reduces the final level of moisture in the finished bread.
Higher baking temperatures also cause more water to evaporate. However, it's usually a trade-off between higher temperatures or longer baking. A good recipe tends to be optimized to provide a slightly crispy crust without over-developing gluten or drying out the bread.
If your bread (or similar baked product) ended up too dry, it's probably because you over-baked it or baked at too high a temperature. If it came out too chewy (glutinous), you might have used too strong a flour or not enough fat/sugar.