Daisy's list is pretty exhaustive but I had one additional test performed that just might have been the "fix" for me after lots of failures with proven donors.
You may want to consider an endometrial biopsy that is performed by Dr. Kilman at Yale medical school. Your RE's office can send the samples. There are two biopsies, one during a non-medicated cycle and one after you have added estrogen and progesteone just as though you were preparing for a transfer. In my case, the medicated one showed abnormal cells. The theory is that some women react to the added hormones in such a way that would impede an embryo from attaching. With that result my protocal was adjusted to be a lower and slower introduction of progesteone. And it worked - times two - both embryos took. After years of failure with "perfect" and genetically screened embryos I'm in shock.
PS I didn't do genetic testing this time because I couldn't afford the cost, which is why I transferred two and now I have twins on board. My preference would have been to transfer one normal blast. Oh well!