Take the smoke unit out and take it apart. Take out the element and the felt underneath the element. There is a hole in the bottom of the cast bowl that needs to be open. It gets clogged with pellet material. Scrape all old pellet material out of the bowl, and install a new felt lining if need be. Make sure the piston is free to move up and down and does not hang up.
Once it is all cleaned, reassemble the smoke unit and try it. It should puff normally.