Whats the gap between the blades and the cutter bar? is the cutter bar worn? are the blades set in the drum too close and the cutter bar up to far?
try setting the blades at 3-4 lines out and then set the cutter bar to where the blades just do clear it, about a playing card's thickness clearance betweeen the two.
the blades need to hang out of the drum so that when they come around, they make contact with the wood and pul it into the cutterbar. most times I hear of this problem its that the blades are set in too far, then the cutter bar adjusted up to meet them. if the blades are set back up in the drum too much, the chipper will nibble at the wood and make good chips, but not grab it and pull it through very well.
blade sharpening angle should be @40degrees
-Ralph