Well, if not even dashing works then perhaps you cannot jump over that certain wall/gate. I finished TP a while ago so I'm not sure but I think that there are some gates that you cannot jump over with Epona.
I know you already figured it out but in case someone else needs a tip...
You cannot jump over all fences, only those your are supposed to jump.
I found that you cannot press dash while close to the gate, you have to give yourself some room to speed up.