Its possible that you just didnt ovulate this time round, are you still doing the opk's? Hows the temping?
Sometimes the body gears up to ovulate and just doesnt for one reason or another. If its the case and its a one off its no reason to worry as its "normal", of course if its a regular occurance then you want to see your doctor for some tests.
Also some women just dont get on with OPK's!
Ive seen you've had a positive so hopefully your temp will go up now *fingers crossed*