AF is triggered by a drop in progesterone. Typically, this would mean that you had to have a rise, but it's completely possible to continue having the hormone rise/drop bit without ovulating. Someone with a regular 28 day cycle, in a year, with only actually ovulate on 9/12 cycles. But they wouldn't know, because AF would still arrive. Not sure if that helps?
Lou, I think there used to be someone on here who did ovulate late in their cycle, despite taking clomid...had a positive opk on CD13, but then actually ovulated on CD21. So I think it's completely possible!