First time Ovulation Strips

I just miscarried our first baby in December, I had my first period since mc a couple weeks ago, and I am trying these Ovulation test strips now. I am on CD 15 and still no sign of the lines getting darker. Am I doing something wrong, or can you ovulate past 15 days. My period is usually spot on to the app, and according to the app I should be ovulating today. Here are some pictures of my tests.