Thank you for your question.
I am sorry you are having a problem with the chipper. If the unit will not chip up branches, I suspect the chipper blades are packed with wood or the blades are dull. I recommend checking the blades. To check the blades, remove the spark plug wire and the branch chute. Then pull the starter rope to rotate the impeller until you see one of the blades. If it is packed with debris, you can use a screwdriver to dig it out. If the blade is dull, I recommend replacing the blades. The blades can be order from the following link.
Replacement parts for Craftsman chipper shredder
I hope this is helpful. Please let me know if you need further assistance.
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