Hi there, Hope4ABaby.
I'm sure you've done your research, but I have happened upon Dr Sher's blog and found it really useful. It's IVF Authority. You can search for the articles there on the main factors affecting implantation, and this includes possible hidden sperm DNA factors. Has your husband had a sperm DNA integrity test (which tests for DNA fragmentation)? I find this stuff really tricky, but perhaps you know all this already?
It's my next question for my FS.
If you have any more knowledge than I do, I'd love to hear it. Let us know how things go with your next appointment.