I had the same problem, strumming down was easy but coming back up I just kept catching the bottom string or missing the strings entirely. I found it was down to my wrist position, because I had only ever strummed downwards I had got into the habit of holding my wrist at a really strange angle where the plectrum was almost aiming at the ceiling, this made strumming down easy but strumming up impossible.

Now I have my strumming hand a lot more neutral if you like, it stays in pretty much the same position for strumming up as well as down with only a slight natural bending of the wrist.

I think it's an area that is really overlooked, you almost take it for granted. Because you can strum down you just assume that you can strum upwards. It took a while to sort out but now it's just automatic.