Fluctuating ovulation tests normal?

Hello all. It is my first time trying to track ovulation while on letrozole. Started super light. Got a test that was about a shade lighter than control line. This made me happy ! Tested next day and line was super light. Next day back to being a little darker. Did I already ovulate and miss it? Is it normal for it to go back and forth so drastically? CM is also non existent. Is this most likely a failed round of treatment?