I have jenny that I was hoping would give me a hinny, but none of my stallions will breed her. She has been with three different studs, and nothing.
I was doing some reading on this subject, and besides the fact that not many stallions will even attempt to breed a jenny, the other 'problem' is that the conception rate of stallion to jenny vs. jack to mare is very different. The article that I read stated that there was something like a 80-90% conception rate when a jack covered a mare, but that even when you could get the stallion to cover the jenny, you only had about a 10% chance that the egg would get fertilized. So, if you combine the fact that most stallions won't breed jennies with the fact that they have a low conception rate when you do, it explains why it is so difficult to get hinnies.