I've been using OPKs for four cycles now and always get a line on CD16. The lines aren't very dark though but because they always appear at the same time of the month I've been taking them as positive even though they're supposed to be darker than the control line. BUT, this month, I didn't get much of a line at all on CD16, then tested again today (CD20) and got an immediate very strong line.
I don't think I've tested this time of the month before so maybe I would have got the dark line on CD20 the other months too? Or can the day I ovulate change each month? I didn't think so...