Positive OPk but no mature follicles

I’m on my second cycle of letrozole. I had an ultrasound today on cycle day 15 which my RE couldn’t see any mature follicles or the sign of ovulation.

But I did tested positive on opk on cycle day 14. I have also attached the picture of my positive opk. This cycle was a bit weird, I usually ovulate on cycle day 18-19 and that’s what happened last cycle too(I took letrozole from CD5-10).

This cycle I took CD 3-7 and tested positive on cycle day 15

Has anyone experienced this? I do not have PCOS.