There could be a lot of things going on here. The place I’d start is with your grease. What are you using, and how much? The only grease you should be using in a hydra hub system is DuMonde Tech freehub grease - it’s very thin, and even so you really only need a tiny amount. Anything heavier could be causing slipping. You may also try using DuMonde Tech freehub oil instead, which will make the hub slightly louder. Again, you don’t need much, a drop on each pawl will do.
Yes, there are other oils and greases in the world. Not all of them are appropriate, and these two are the only ones spec’d by the manufacturer. I use both on Hydra hubs in the midwest, and haven’t heard of any skipping issues. But your issue may also be something different.