It is possible for OPKs to not detect ovulation. Your LH surge could hit its peak at night and not be detected on a morning test, either before or after. Your LH surge could not be high enough to be detected on a test. Those would be the most common reasons. You could try taking the tests 12 hours apart instead of 24 and see if that helps. But the best thing to do is to chart your cycle as well. Cervical mucus is a good indication of fertility to tell you when might be the best time to conceive. Basal body temp will tell you for sure when you've ovulated so you don't have to keep guessing.
Also keep in mind that it's possible, too, to get a positive OPK and still not ovulate for some reason. So it's still best to chart other signs in addition to using OPKs.
at 10:56 PM on Feb. 21, 2010