WTBMOM - the thing that we also always have to keep in mind is - even if what looks like good quality embryos are transferred (at 5 days blastocysts or at 3 days 8 cell-embryos) at this point, we cannot tell what the genetic make-up is. Yes, we have fertilised eggs - but even in "healthy" people who can get pregnant naturally, many many cycles will pass, where the eggs get fertilised and develop, maybe even start to try implanting - but due to chromosome issues, will not survive.
That was likely the case many a time with us. And it doesn't have to be, because there is something wrong with the parents, but simply, because in the development, something goes wrong. Our doctor told us about a woman, who had had a child long long ago and then was having trouble getting pregnant again (had several chemicals and miscarriages). So they tested the parents genetically and it turned out, that on two of her chromosomes, she had a genetic mutation - which didn't affect her, but was perhaps playing a role when creating offspring.
So they decided to try IVF and do PID (pre-implanation diagnostics). They ended up having 5 (I think) blastocysts, which they then tested. The first showed a genetic mutation on 3 different chromosomes (one of which the Mother also had, but two she didn't). The second also showed genetic mutations - on 5 different chromosomes (another of the Mother's but also 4 additional ones). The third and fourth embryos showed genetic mutations all across the board on several chromosomes - NONE of which the Mother had. The fifth was fine, was transferred and ended up being their take-home baby.
But this only goes to show, that when creating a new life, things can go wrong and when we think we are transferring embryos that seem to be perfect, they might not be - and therefore, they just arrest in their development at some point.
We can get only so far with science - and then Mother Nature does the rest. Our doctor was convinced, that it was just a matter of time before we would get pregnant - but he just couldn't guarantee how long it would take. He did (after we switched to him in November 2012) tweak the treatment, suggested supplements etc. and with each try, the results got better and better ... but without doing PID, you can never be sure if the embryos being transferred are going to stick ...
I mean, I am healthy - my husband is (excepting the lack of sperm / the lack of motility when there are sperm) also healthy - no genetic issues or hormonal or immune issues found ... so after fertilisation, there was no reason why it shouldn't work : except for nature ...
Maybe trying pre-implanation diagnosis would be something you could try (after testing everything else) - it definitely ups the chances of transferring and embryo that is likely to survive ...