I'm confused

Teemo Troll • I tell it how it is
How does this whole ovulation thing work? The app says today I have a 33% chance of becoming pregnant because I am ovulating. I took and ovulation test this afternoon and it said negative. Last month my ovulation test was correct with the app now it isn't. Am I tracking it wrong? I wait until I'm close to the date the app says to test for it.