Smoking time for 6 racks (18 lbs) of baby back ribs

I rarely ever recommend boiling the heck out of meat. You are washing away all of the flavor. Remember water is a solvent and remove everything from the meat if it is left to boil long enough. For the most tender ribs I would recommend a braise. The slow, low, moist cooking of a braise is perfect for breaking down connective tissue in the ribs without drying them out or washing away natural flavor. Even in smokers I haven't had great luck with baby back ribs as they have a tendency to dry out.

Here is a easy to follow braise for ribs by Alton Brown video, 1, 2. It's a decent place to start and work on your preferred methods from there.

Please avoid boiling your meats, you will be so happy with other methods, even if they do take a bit longer.

The recommended cooking time is only a guideline. There are a variety of factors that influence cooking times in a substantive manner. For what it's worth, water in the water pan is not one of them.

The purpose of the water in the water pan is to act as a heat sink, in order to prevent temperature spikes. Anyone who claims that it helps keep what you are cooking moist should be summarily ignored. One of the more likely scenarios is that the thermometer is either inaccurate, or it is not an accurate reflection of the area in the smoker where the meat is being cooked. There are no guarantees that the temperature you read at one point is constant throughout the smoker. In fact, it is almost a certainty that the heat sink (water) will cause a cooler area in its immediate vicinity. This is pretty common in the Weber Smokey Mountain, and is why I don't use water in mine.

I would suggest some things:

1. Check the calibration of your thermometer (get a temperature reading with the probe in boiling water).
2. Put a thermometer (an oven thermometer will do) in your cooking area, to get a sense of any discrepancies between the lid and the cooking area.
3. Crank up the heat, if necessary (250f is still "low and slow").
4. Experiment with not using water, to see if that makes the heat distribution more amenable to cooking in your smoker.

Now that you have some cooks under your belt, the time is about right to tweak your methods to your liking.