Honestly it's hard to say what's wrong without video of your riding.
Are you trying to use an edge or turn while on the box? That's usually the common reason people fall on boxes because they aren't treating the box like a frictionless surface.
As long as you keep your body weight balanced and you keep your base flat against the box (no turning at all, even if you aren't lined up correctly you have to just go with it) you shouldn't be falling.
If the box is a little sticky you could try going faster. Sometimes going slower makes sticky boxes more sticky because you have less momentum going forward.
One other thing is to check your edges for burrs. This can cause you to catch on boxes if you have big burrs along the edge of your snowboard.