Ovulation tests negative...

According to this app today I have a 33% chance of getting pregnant...the highest day. But I've been taking ovulation tests morning, noon and night for over a week now and they're still coming back negative. This is my first month of not taking any contraception, could this have an effect on when I ovulate? I know the app isn't 100% and as its my first "natural" cycle it may not be 28 days as the app "thinks". Any advice?