Ovulation tests

How accurate are ovulation tests? I’ve been doing this for about two weeks now and have never had a clear surge in lh. The line has been close to the same shade as the reference line but that was always a mid day pee after I drank water. I would take another one later in the evening and it would be substantially lighter. I have never had one go darker then the reference line.

Could I have still ovulated and my tests not been accurate? I’m trying to avoid pregnancy.