James,

Isn't your rig a syphon type gun? The supply air (max you can get) passes through a venturi and pulls the blast material in by suction. The material must be dry and fine enough to pass through the selected nozzle. Does the unit indicate it will work with walnut shells...they sound pretty coarse to me.

If you hold your finger over the nozzle and give a blast of the trigger, it will "backflush" the suction line and clear any obstruction.

What do the instructions say...I know you read them. I'm still amazed with the walnut shells deal.

Stuart