Why haven’t I O’ed yet?!

Hello all, so I have been testing since CD12 (very top dollar store opk) twice a day for O, and today is now cd21 (bottom opk) and the opk’s are getting lighter and still no positive...I usually ovulate on cd 17, although I did ovulate early last month on cd 15. I am pretty regular and the latest cycles I usually get are 31 days...my lp is always 14 days. I thought on CD12 I was getting super close and for a couple of days they were pretty dark, then all of a sudden they were light. Does anyone know what could be going on? I’m so confused...