OPKs not getting darker

Laura • High school teacher, rescue dog mom, army wife
This is my first month using OPKs. I'm usually 28-30 days like clockwork but last month my cycle was 45 days for no good reason. I'm currently at the end of CD15 and my OPKS aren't getting any darker. It actually looks like they're getting lighter. I'm so afraid that I don't ovulate but will keep testing. Glow puts my ovulation day at CD20. Weird. Anyone else have this experience?