Confusing OPKs

I've been using OPKs twice a day and I can't get a clear positive. One day, I'll get a pretty dark line, but not darker than the test line. Then later in the day, no line at all. Then a few days later, I'll get another darkish line. It's been like this for several cycles and I still can't get a clear read. I tried taking my bbt everyday, but it's so hard to remember before I jump out of bed. lol

Does this happen to anyone else? I just want to know if I'm ovulating. I'm so confused!