OPK help!

I've been using OPKs for the past two months, and have never had a clear positive read. This month I'm trying Clear blue digital 20 pk, and no smiles yet! I have a 31 day cycle, starting testing cycle day 12, once with FMU and one again in the evening around 7. I'm starting to worry that maybe I'm not ovulating? periods are regular, age 29 TTC since January. Any advice or experiences using OPKs I would love to hear! Thanks!