Your pile height may be too low. Ideally, the gap between the top of the pile and deliver roll should only be ½ to ¾ of an inch. Raise the position of the feed down sensor so the sheets can fall freely into the stacker.
If this does not correct the problem, try adding air at the pile. By directing air out across the stacker, the sheets float out over the pile.
At the same time, air can also be directed down into the stacker, forcing the tail end of the sheet down into the pile.
