Avalon Astoria insert: All ash drafting into exhaust

[enigma_rva]

I've been burning a pellet stove for 15 years now. Now have an avalon astoria bay insert for 3 years. Very suddenly, almost all ash is getting drafted into the exhaust with almost nothing remaining in the burn chamber. This is occurring even if I keep the damper closed down to almost nothing! I've can run 5 or 6 bags and next to nothing is in the burn chamber. I am fairly confident the damper is working properly because I can keep the air flow adjusted to a good burn. I figured if it was broken (wide open) I'd see the high flow rate and the stove would burn itself out. I have not pulled the stove to disassemble completely (trying to avoid since it is now winter). But I am having to pull the unit far enough to vacuum out the flue elbow at the back of the stove. Any thoughts? Thanks.