Accuracy of OPKs?

I've been TTC for 8 months now, using OPKs for 5 months. I just took an opk and it was negative - again. My cycles are regular and so I should have gotten a positive opk 3-4 days ago. Should I just stop taking the tests at this point? I'm wondering how common it is to NOT ovulate on a given month. I've gotten a positive result the last 5 months, and always at the same time. Any similar experiences?