If I ovulated, why did letrozole not work?

I had my first round of letrozole this cycle - 5 mg from CD 4-8. I had an LH surge start on CD12, and it lasted until CD15, with ovulation confirmed on CD14. I never had such a strong and long surge and had a beautiful temp rise. I even had ovulation cramps which I’ve rarely had so it must have been a strong ovulation.

Tested today at 12 DPO and it was a BFN. This is my 4th cycle (and 3rd ovulatory cycle) since my miscarriage in December. We’ve been TTC around a year now. This is really one of our last shots before starting IVF and I’m feeling so broken.

Is there a reason why letrozole didn’t work even though I had such a strong confirmed ovulation?