I'm supposedly fertile but neg ovulation test?

According to Glow, tomorrow is my most fertile day, but my ovulation test keeps coming up negative. Am I testing the wrong day or what? I didn't use ovulation tests the first time around so they're new to me. The instructions said to start testing CD 12 and its CD 15 and still negative. My cervix is high, soft, and open so I thought I'd be ovulation soon. I'm so confused. Is there a trick to this lol!