Hard to tell without seeing - offer the following thoughts:
- Are you cutting into, or with, the grain? Look at the edge grain and feed so knives won't tend to lift grain.
- How deep are you cutting? Sometimes reducing to 1/32" or less will reduce this to a non-problem. Takes longer but cleaner
- Is the grain straight or kind of gnarly. If gnarly, try wiping lightly with a damp rag or sponge. Sometimes this softens the wood grain enough to be more forgiving. You'll have to wipe the bed down afterwards to remove water. Let the machine run a few seconds to sling the moisture off of the knives.
By the way are you planing with a jointer or surface planer. If jointer, feed as slowly as practical.