Hank,
When you say "mulch insert" are you describing the blank plate covering the discharge? My only thought goes to the baffle system underneath. There are two baffles, one to be used for side discharge,(sort of blocks cut grass from blowing/moving towards the rear of the deck. The second baffle is for rear discharge and is mounted towards the front of the deck leaving the rear open towards the rear discharge. I imagine the "wrong" baffle might result in better mulching in reverse.
MIke