u/Over-Juggernaut7560

My camp chef pursuit has always started to back burn up the auger at about the 3 to 4 hour mark of a smoke. Any ideas guys? I've gutted it cleaned it crazy the vents at the back aren't blocked it did need a sheer pin once. I've tried lots of brand pellets gmg lumberjack etc all do the same. The fire pot fills and it starts going up the auger. Could it be the fan or just a shit unit?