What was the reason for the failed implantation ?

One of our patients sent me this email.

Doctor,
We were hoping for some detailed response from you which would have given us some comfort as this time we are very very shocked. We believed that the use of donor egg would resolve my egg quality issue but things doesn't seem to have changed.
I clearly remember in my first cycle, at the time of my ET, we had only one embryo left for transfer and which was at morula stage. You had explained the reason for low number of embryos and slow growth was due to the poor quality of my egg, which had failed to provide energy to the embryo to develop. Even when implantation had failed at that time, you had said the reason was poor quality of the egg. So, does this mean that in this cycle, the eggs which were used were also of poor quality which is why we received less number of good quality embryos and also failed implantation?
Another thing which we also recollect from our first cycle is that we were worried whether it was a sperm issue for failed implantation and whether we should we go for some tests like PGD. You had explained that egg quality is generally responsible for poor embryos and that poor sperm quality do not affect embryo quality after ICSI. Hence, the requirement of any tests was also ruled out.
My uterus and endometrium lining was also perfect during this cycle. So what could be the reason for failed implantation this time? This is a big question. We made the toughest decision of our lives to go with the donor eggs but still the question remains unanswered is something which is not acceptable and hence we look up to you for your fine judgement and expertise.
Now, after having failed implantation from the one and only top quality blasto, we don't know how those two remaining frozen embryos will help if its not from a good quality donor eggs because we don't want to build any hopes for something which is having very little chance of success. 

This was my answer.

These are good questions - but no one in the world can answer them, sorry, so you will have to accept the fact that  reproductive sciences have limits
There are certain things we can control - and lots we can't, and we need to learn to live with this uncertainty and unpredictability
Yes, we could lots of useless tests for you, but none of them would answer any of the questions you are asking
Please have realistic expectations of what medical science can do for you
I would suggest you might want to get a second opinion from another IVF doctor, so you can reassure yourself that we are providing you with good quality care
We do want to give you a baby, which is why we  reduced our fees for you - but we have no control over implantation, which is an in utero biological process
Yes, I would suggest you transfer your frozen embryos, but there is no certainty they will become babies. However, if you do not transfer them, it is 100% certain they will not become babies !

Please remember the Serenity Prayer
God grant me the serenity to accept the things I cannot change;
the courage to change the things I can;
and the wisdom to know the difference.
I did try to call you, and will be happy to answer any questions, but please understand that we don't always have answers - I wish we did !