Won't burn on low

My friend called me to take a look at his pellet stove. It lights fine and will burn without a problem when it is set on medium or high heat. However, when set on low, the burn pot fills with pellets and the fire goes out. I checked and cleaned everything I could think of. The burn pot looks fine, vacuum switch and hose are operating perfectly, and vent piping is clean. Any suggestions?

If you provide the make and model of the unit that will help us help you....

Try checking all gaskets and door seals. May want to increase the trim on the exhaust motor or increase the air intake.

Lopi-heritage bay.

It's an insert

Did you clean the termination cap, all ash traps, the combustion blower and the cavity it sits in?

Make certain that all of the burn pot slots or holes are clear, the area above the burn pot and to both sides and around any heat exchanger tubes. Hopefully no one has hit the damper etc ...