What I have found is that the holes in the chime bars are not all drilled in the correct locations. They bind on the mounting screws due to the holes being slightly off center. It puts a bind on the grommets and the brass inserts.
In some cases I've had to either ream the chime bar hole out a tad bit bigger or remove one of the brass inserts from a grommet.
Also, be sure you are not tightening the lock nut down too far. The chime bars need to move.