ovulation

I've been doing ovulation tests for a while, yesterday's was the darkest yet but not positive today I've done 2 and they're both really light. Any clues as to why they're not getting positive. It seems like I'm not ever gonna ovulate. I know I can get pregnant as I have 3 but we're trying for our last and every time we think yay the test tomorrow will be positive as today's is the darkest it never happens. Anyone know what's going on?