It's likely that they pushed some flyash accumulation into places in the pellet stove creating a blockage. Unfortunately a proper disassembly and thorough cleaning is the only way to be sure the blockage(s) are removed and that it will be safe to operate. This is why I don't like pellet stoves and if you're looking for an automatic solid fuel heater (not a hand-loaded woodstove for example) I would always reccomend looking at coal stoker stoves. automatic coal stokers look much like a pellet stove with a few benefits; they require much less frequent attention and cleaning and in most cases they are a much more straightforward design - easier to clean when it needs it. Additionally, coal in most areas (ie eastern US) where it's available will cost about 1/2 what a pellet stove costs to operate - coal is much less expensive for the amount of heat produced, the fuel is about twice as dense as pellets.