First IUI failed

With the first IUI everything looked good except the lining it was 3.5 mm instead of 7 and above. I took clomid and ovideral and estradiol to thicken my lining. When I got my period recently I went in for baseline for the second IUI. The dr said the clomid was making the lining situation worse so they are putting me on letrozole 2.5 mg. Everything looks normal my blood work, AMH and my husbands semen is normal just the lining is thin. Has anyone had this problem with their lining being thin? If so what did you do to help your lining be thicker?