The general rule of thumb here is 3 weeks primary, 3 weeks (@~70 degrees) bottle for standard beers. Your kits probably say something like 1 week primary and 2 weeks bottle. You will have much better beer if you wait it out. If you don't let the yeast eat up all the sugar, then you will be undercarbed. Other things that can contribute to it are not priming with enough sugar. Since you are using kits, I am guessing they came with priming sugar in the appropriate amount. If you don't want to carb to style, a general rule of thumb is 4.5-5 oz of priming sugar to 5 gals of beer. If you are bottle conditioning too cold, it can take longer to fully carb.
If after three weeks you still have poor carbonation, I have heard that you can flip all your bottles upside down for three days, put them back rightside up for three days (causing the yeast to be in suspension), and it will help them eat up the rest of the sugar (assuming you primed with enough).