I am so sorry. After my first failed DE cycle, I had my RE do immune and clotting testing (lupus anticoagulant, anti-cardiolipin antibodies, beta 2 glycoprotein were the items that my RE was looking at the most). There are specific panels which can be run- and this can get very complex, very quickly. One of my tests initially came back a little high; I went to a hematologist for even further follow up.
If your lining was looking good before transfer (and you've had either a saline sonogram or HSG prior to cycling), I'd get those tests next.
I also went to my GP and had a physical for everything under the sun. My thyroid was high normal and vitamin D was low. My RE worked with me to correct these prior to my next transfer.
We did go to get a second opinion- it was re-assuring to hear that our cycle was managed properly from a medical perspective, and the second opinion RE believed that it was just a matter of trying again.
GL to you! Some of the ladies on the DE check in have had much more complex testing than I and could also provide thoughts on what to do next.