If your animals are cycling naturally, they may be getting too hot during their active season, especially your adult males. Try keeping them at 78-80 maximum degrees F. during their active season for a couple of years and their fertility may likely improve significantly. Also, avoid extreme cold and humidity during brumation. Another possibility is that your adults need to be put together earlier in the season and remain together (or repeatedly introduced female to male) for a longer perid of time from April to June. This longer-term stimulus often induces increased gamete production in both genders leading to more successful matings.

I hope this proves helpful. Good luck.