Implantation issues h

Hi
I’m 27 years old with lean pcos.
I have had 2 good quality blastocyst transfers, both giving me faint BFP on 5 days past transfer which slowly goes to negative by 12-13 days past transfer.
I don’t have endometritis, no hormonal issues eg thyroid, blood clotting normal, I have had a hycosy which showed normal uterus. I have always had 8mm or more lining trilaminar. I am now going to do NK cytotoxicity blood test and micro biome test of my endometrium. Do you think these tests are appropriate? Is there anything else I can do?