Never any line on OPK's

St
I stopped Depo Provera in September of last year. I'm just now starting to have periods again, a total of 5 since stopping the shot. For the last 2-3 months, I've been doing the OPK's to try and help me find my ovualtion days. In total, I've only gotten one faint line, and that was 2 weeks ago. It never got any darker. I assumed that I somehow missed my window. 
​I've recently read that women always have the faint line on the tests, and that they just wait for the darker line. I was all crazy excited over my FIRST FAINT line! Should I always have that?  Or is it as I assumed....no line at all until the hormonal changes at onset of ovulation? I'm so confused, and now I'm worried that something isn't right with me because of never getting the damn line on OPK. This line thing is making me crazy! lol