What are your yeast pitching and fermentation practices? Do you use yeast pitching calculators (Mr. Malty, Brewer's Friend)? How long do you ferment? Do you use priming calculators (Brewer's Friend, Northern Brewer, Tasty Brew)? On the calculators you should enter the highest temperature your beer reached during fermentation, not the current temp. What are your bottle conditioning temps? If the bottles are in a cool area try moving them to a warmer part of your house. As others indicated, measuring the priming sugar by weight is preferred.