If it's a wide flat box I have this exact same problem and I still can't figure out how to correct it. If it's a down box I can use the inside edge of my leading ski (the technique others have described) and it works pretty solidly, also seems to work on flat boxes if they are narrow.
Rails for some reason are way easier to rotate off of, maybe it's because your ski camber grips the rail better than a box? I'm still new to this so I don't know. Don't give up, and maybe try some rails or tubes that are low to the ground since they seems easier to rotate off of.