Starting fine . Avalon 900 PI, then going down hill

I have been burning all season starting Nov 1st and just recently (last 20 days) have an issue. I came home from work and the burn pot was loaded with pellets and a super lazy flame. I have no idea what I did that would have caused this ??? I did go to a different brand of pellets but all season I have bought from the big box stores and the stove has handled that fine.

Here is what I have done :
- Complete cleaning
- Pulled fly trap vents and cleaned
- Pulled heat blower motor and checked intake for obstruction
- Pulled exhaust 'T' and cleaned out

Now after everyone of these checks/cleanings I re-assemble clean the glass and get about 4-6 hrs of decent burning then it all goes down hill and the pot starts clogging, glass gets real dirty and the flame gets lazy.

I did a search and couldn't if anyone has encountered this - Starts fine and then goes down hill.

Thanks for the help in advance.

O and I have a Avalon 900 PI - Its an older model but when I installed it in the fall it worked great

You think I need to pull the 20+' of stove pipe? I have 20' of flex off a T that terminates to a cap - Pull the flex?

Per the manual I am allowed 33' of vertical with less than 10' of horizontal - I am going to pull (again) tonight and clean it ... Will post the results.

Great catch .... I didn't realize this, I actually have closer to 18' of vertical but I will def make the change when the burning season is behind us. Right now I just want to get it back to the burn I had in Nov. to get me to April.

Big thanks for Bioburner - Pulled the T last night and went at the pipe with a brush, collected about 2-3lbs of ash out of the pipe (prolly a lot less just looked and felt like that). Put the exhaust back together and fired it up and its running like a champ

OK ... I am at wits end here ..... As you can read from the above posts everthing that I have done to the stove to get it to burn like it did at the begining of the burn season - HOWEVER - I just can't
I cleaned the exhaust/termination piping and the stove worked great, just like it did when I first installed it in the fall but after a full day and a 1/2 of burning its back to a loaded burn pot and lazy flame UG !

I have no idea what the issue is, like I have said all along it starts fine and I get 6 hrs of good burn then it goes down hill .....

Any/all advive ideas are welcome

Does "high ash" translate to a small cake of clinkers built up in the burn pot ? There is barely any ash in the inside of the stove BUT as stated the burn pot is clogged ..... Pellets?

Latest update ........
- Burning only 'premium' pellets
- Door gasket
- New Comb/Exhaust motor

Stove starts great and by the third hour the glass is completely black, flame getting lazy and pot starting to fill, I have to keep reducing the auger feed until it can't keep up and shut down and reclean.

This is now becoming a mission to fix whatever is causing the stove after 3-4 hrs of good operation to go down hill ????

Does anyone have any ideas ??