I am having some of the same problems. I have tried several thing and the following seems to work best.
First, insure that the bottom of the piece you are coping the piece to fit is at the proper angle on the wall. Take a piece of the crown molding and place a square against the side that meets the wall and the sside that meets the ceiling, take the distance the bottom of the molding will be from the ceiling. Then ensure the bottom of the crown molding is x distance from the ceiling. This ensures a proper alignment of the molding, one that will allow a 45 degree miter/coped angle to ajoin it.
If there are still problems mating the two, use a wood rasp, one side flat and one side rounded rounded, to take away any excess material. Any minor adjustments needed can be filled with caulking. I hope this works for you.