I dont use OPKs as I've had a cycle where they were positive, but then i didn't ovulate so i spent 3 weeks wondering why AF was late and hpts kept coming up negative. It is possible your just missing the surge, it may be happening in the middle of the night when your not testing for example. And for some women the surge could be over in just a few hours instead of building over a day or two. If you want to know if your ovulating, you need to chart your bbt and look for the temperature shift, wont tell you beforehand your about to ovulate, but it will tell you afterwards if you are or aren't.