Hi bubbat
It's great that there are no obvious issues for you, but I get that's very frustrating when there's no obvious problem to fix!
Sounds like your DH is fine, and it's a great sign that you're regular and o'ing. Other things they could check might be:
- ultrasound to see if your lining is thick enough to allow implantation after o
- Checking for fibroids, blockages or other issues with your tubes(the hsg should cover that)
- Antibodies in your CM
- Your lining of your eggs may be too thick to be penetrated by the sperm (i think this could only be checked with a round of ivf)
- Eggs may not be viable, or genetic issues once fertilised (again, I think a round of ivf would be needed to check this)

Hope that's of some help - Good luck!