Wayne "chuck n duck" knife/rotor/bed plate adjustment

[RollinK]

I have a great running Wayne "Chuck & Duck" chipper that I've had for several years. Makes a nice chip but not very aggressive on pulling in 2"+ limbs. Pulls in smaller limbs great (less than 2") but anything over that it cuts the end of the limb at the same angle as the rotor and stops feeding. Knives seem to be at the correct protrusion from the drum (as per the manual), 6 sided bed knife gap is correct and edges seem reasonably well defined, could it be front to back bed plate position adjustment? I was thinking if I moved the bed plate forward, it would enlarge the gap between drum and bed plate and allow better/ more aggressive feed in on larger material. Anyone have an answer? Thanks !