HSG clear, SA perfect, what now?

So, I’m 23 years old with a BMI of 18.5. DH (dear husband) and I have been TTC (trying to conceive) for 16 months now. Hubs SA is great. We’ve done 3 cycles of clomid, none of which produced a 21 day progesterone greater than 9.8. I just did my first letrozole cycle (2.5 mg CD (cycle day) 3-7). I was becoming concerned that I had tubal factor infertility due to an infection I had years ago but just had an HSG 12/31 and it was all clear and my doctor even said I had a perfect uterus and tubes. He said we would wait for my next cycle and increase the letrozole dose. Anybody had any luck with an increased dose of letrozole? I’m beginning to feel hopeless...I have heard of people conceiving with a lower progesterone level than 9.8 and carrying their babies to term.