Give it routine maintenance like the tip below for glass smooth results board after board.
Knives Dull

When knives dull, they often tear the wood or pull chunks from the surface, so replace or sharpen them at the first sign of dulling. Don't bother sharpening double-edged, disposable planer knives; it proves too difficult to get a consistently sharp and straight edge on all knives–which must match perfectly. Keep a spare set of knives on hand to minimize downtime.