Not knowing anything else about you or your cycle it would impossible to guess. But the most likely cause would be immature eggs or technique.
I think you should ask your doctor for his/her perspective on the reason and if the answer is not one of the above, then I would get a copy of the records and get a different perspective with all the information available.
Arthur L. Wisot, M. D.
Reproductive Partners Medical Group
Redondo Beach, California