Maybe you have polycystic ovaries. They will have irregular cycles because follicles donot grow to mature state by 13th day, so follicle doesn't rupture to release egg on 14th day. Hence, pregnancy will not occur. Endometrium is thin coz there is no mature follicle. Letroz 2.5 mg may not have worked. You may require higher dose of letroz and also HMG injections from next cycle. Weight reduction(if heavier) helps in follicle development.

Endometrial thickness is less and no dominant follicle