If you read much of the writings of the hunters. Baker for example, you will find that they did not greatly care for the bullets in MLs. As already pointed out they are not noted for staying where they are seated either.
If you are cutting patches it could be at the muzzle as Daryl suggested or you could have a wire edge on one or more lands. If you run a very tight patch up and down the bore it might give a clue. Also starting a ball on strip of patching large enough to grip then pulling it out again should show any problems with the crown.
A good scrubbing with 0000 steel wool should remove any wire edges or burrs from the bore this if this is the problem.
I generally use a cloth patch with the stell wool over it making it as tight as feasible.

Dan