Late ovulation on Letrozole causing low numbers on progesterone test?

Using opks this month, I got my peak (solid smiley) on cd 18. When I went in for my 21 Day progesterone blood draw, it came back at 6.2, which is too low. My doctor said that I didn’t ovulate this cycle. When I asked if the number could be low bc I O’d later, he said that my opk was just positive bc I was gearing up for next cycle and it was too late. Does this sound off to you? Could he be saying this bc I’m on letrozole? Does that make it impossible for me to O late? TIA! 😊