Hi Inconceivableme, I hope you are well?
I don't have any experience of FET, but I would guess that implantation followed by early loss points to an issue with the embryo, rather than anything you/your clinic did or didn't do.
If I were in your position, I would be asking what grade the 3 frosties were, and what grade the one they put back this cycle was. i.e. was it the best of the bunch, or a random choice? Are they all blasts?
This would help me make the decision about whether to try another frozen cycle or do a fresh one. If the best of the available embryos didn't result in a successful pregnancy, I would question whether a different, lower grade one would.
Remind me again, how old are you, and what is your AMH? Would you be willing to try another fresh cycle or are you committed to the frosties?
Also, I would ask if PGD might be an option...I'm kicking myself for not doing that this cycle, and if I ever had another I would def do it...