Pellets not burning, backing up.

I have a Harman Invincible Insert, after I start it, within the first hour pellets start backing up and start to choke the fire. I have replaced the combustion blower and the thermocouple, also cleaned the exhaust tubes, pipes to make sure they are clear. Still having same problem.

The Harman Invincible has 2 ash traps inside the firebox. The covers can be removed by unscrewing the thumb screws. They must be thoroughly cleaned including the chamber that that connects them across the top of each one. You can use long brushes for the vertical chambers, and then I recommend blowing them out with a strong compressor to fix your problem.