Glow stopped predicting my ovulation date.

So I have been using opks. Im on cycle day 28 and still havent gotten a positive. Glow was bumping my ovualtion date forward every time I logged the negative opk but now it has just stopped. Even with me still logging negative opks. I honestly dont think I’m ovulating at all.. is it normal for glow to just stop estimating? Also why would I not get a positive opk? I’ve gotten some opks close to positive but nond clearly positive! I have an appointment with an ob.. has anyone else had a similar experience?