I never had success with OPKs, but that's more to do with frustration with my PCOS than anything else.

For some women, they'll get a dark line, that's not as dark as the control line, which technically is still a negative (if you take the OPK instructions at face value), but it's as positive as possible for them. It may also depend on the time of day that you are doing your OPKs - morning? afternoon? middle of the day? I've heard of women having a very short LH surge consistently during one part of the day. If that surge occurs in the afternoon, but you test in the morning you won't pick it up, especially as it may have already subsided before you get to the test the next day.

If all the physical signs are pointing to ovulation, I'd assume that ovulation is actually happening, especially if you are getting darker lines on you OPKs, even if they aren't as dark as the control line. It's also possible that the OPK will turn positive in another day or so.

Good luck!

BW