Simple check (as Joe alluded to) is to check the reveals and make sure that the slabs sit flush with each other and the jambs. That will give you a great idea of whether or not everything is plumb and level. If it is, you can putz around with little things here and there to make it operate more smoothly, but you'll have your work cut out for you because it could very well be a "loose" factory assembly, or some component slightly out of spec. You didn't mention what type of door it was, but this is often the case with cheaper doors. Even high-grade french door units can be VERY finicky to install.