Miscarriage and failed IUIs

I’m so lost and confused. Last May after our 2nd IUI I ended up pregnant and losing it to a missed miscarriage. Then took the medication to pass the miscarriage. After a few month of monitoring, my lining wasn’t going back to normal and ended up have a d&c. This ended up being due to fetal tissue stuck to my lining. We now have proceeded with 3 IUI attempts which have failed. The providers feel that there isn’t any additional testing needed. I’m lost and frustrated that we are unable to get pregnant again. Anybody have any advise or have any similar experiences?