You're not supposed to be able to climb mountains from an direction. There are set paths for each mountain, like trails, that allow you to climb them. Of course you can get around them by climbing up steep hills and such in ways you're not really "supposed to" but if you do that expect a chance of glitching into it or getting stuck.
Basically put, if you need/want to get to the top of a mountain, circle around until you find a path if you are not wanting to risk getting stuck.
Sometimes I find it misleading, sometimes what looks to me like a path takes you so far with a little jumping then you get stuck, sucks when you cant find the path I think.