Has anyone used the First Response OPK & found them any good?

I THINK I ovulate on about day 14, based on CM. However I've tried the OPK from day 12 (meant to start day 11 but forgot ). Two days of NO test line at all, then every day since then (starting day 14) has had a faint line which was weaker than the control line - which as I understand it means it hasn't picked up on the luteinizing hormone and hence I'm not ovulating according to the OPK.

Could it be a dodgy test? Could I be having a cycle & AF, but not ovulating? (have had very regular AF since January despite expressing exclusively). I hadn't been checking basal temperature or anythning like that - is that worth doing instead?

TIA,
Belfie