Just like many others before me, my Dewalt planer started to slip recently. A quick checked confirmed the rollers were dirty, probably from planning a lot of salvaged T&G pine siding that had old Varathane on it.
The following pictures show how to clean the rollers. Dewalt recommends using mineral spirits and that worked fine for me. Note, I also cleaned the pitch off the cutting blades, use extra caution around the sharp blades.
The whole process took about 15 minutes and the planer was back to new working order.