It is possible you missed the surge, but I would keep testing for the next day or so to be sure and of course bd like mad just in case. What brand of OPK are they? I think that some are automatically darker than others.

Up until this cycle I had never ever even SEEN a 2nd line on an OPK and this month I saw two. I tested every day from the start of my cycle and then when they started to get darker I tested 3 times (or 4 times) per day to be sure I'd catch the surge. AND DID!!!

Good luck.

Lisa