My 2008 VistaFlame VF-100 pellet stove recently began to accumulate pellets in the burn pot. This happens after 3 days of burning. They are burned, but a gray clump forms and they build up in the burn pot causing the flame to get lazy and create black soot. I have used the same pellets all season and they have burned fine. I clean the stove every week. The seals in the door and ash pan seem to be sealed tightly. I replaced the exhaust motor 3 years ago. The first lasted 6 years. Does anyone have any ideas? Would it be worth replacing the exhaust motor again? I appreciate any help. Thanks.