Cycle day 17 and still no positive OPK - wtf?

Melissa • TTC 3 years. IVF BFP 9/17. 🥚due 5/18
I used an OPK starting day 9 twice a day. It's day 17 and still no peak - last month it was day 16 and the month before that was day 13. This is my third round of letrozole