Researchers have been working on determining why some embryos won't implant:
The team, from the UK Universities of Southampton and Warwick, and the University of Utrecht in the Netherlands, found if the endometrium - the lining of the uterus - fails to sense chemical signals from the fertilized egg, it silences many of the genes involved in allowing it to embed in the uterus.
One of the chemical signals the endometrium looks for is the amount of trypsin, a common enzyme, the embryo gives off.
If this is not detected, the embryo is not accepted and is left to disintegrate, resetting the cycle.
Read more: http://www.medicalnewstoday.com/articles/266321.php 
The results from the research can help improve treatments. Any thoughts on these findings?