There is a chance you possibly missed the height of your surge in between the two tests. Another possibility was that your pm test could have been diluted by water or anything else you may have drank during the day, or also could've been you didn't hold your pee long enough? Just thinking of all the possibilities. You should still BD for the next few days though, since you don't actually ovulate til like 12-48 hours after getting a positive opk. Have you been temping as well? If so, does it correlate with a temp rise?