Lining issues

I’m 32 & have severe PCOS. I have a history of uterine cancer in which I was very fortunate to have caught it at stage 1. 8 months of immunotherapy using an IUD & now cancer free for 2 years. Since then I have had 2 failed ICSI assisted FETs that have been genetically tested. A 3rd FET that resulted in a chemical pregnancy at 6 weeks. I am now about to start my 4th IVF cycle. I read you mentioning not doing a transfer with a lining less than 8. I have only I have never been able to get my lining greater than 7 (being my best) even with added estrogen (oral, vaginal, IM, & patches) I am starting to think that may be my issue despite what I’ve been told. What may you think is the cause? I’ve seen 2 different fertility specialists & each have said everything has always looked perfect & I should have had a successful transfer, however, 4 years in and no success. I do not understand what may be the issue. Any insight?