Hey Starfish, as Tiffanyandco mentioned, your body can 'gear up' to O and then not O, this can happen a couple of times in a cycle. Some people don't O every cycle either. I think it's much better to pay attention to CM because for some people, OPK's can be unreliable.
OPK's never ever worked for me, my doctor thought I wasn't O'ing but after bloods it turned out I was. I bought a clear blue fertility monitor after that and it works spot on every time. Some people have heaps of luck with OPK's and others have no luck at all. I was one of the ones who never got a definitive answer from OPK's and it just about did my head in.
It's frustrating isn't it