The problem may be getting enough airflow up under and around the
AMNPS to keep it burning well.
Which generation of MES40 do you have? If it's a 1st generation, you may be able to do what I usually do. I also have the chip loader pulled out all of the way, or at least partway, and the chip tray opened up some, too.
And I always like to have some sort of drip-protector above the
AMNPS to keep drips from hitting it and the pellets. The drips can put it out and burning fat makes really nasty-tasting smoke. So it's imperative to protect the
AMNPS from drips, BUT you don't want the drip shield to restrict the airflow too much, either.
Here's how I usually have things set up in my gen 1 MES 40:
That wire rack holds the
AMNPS up just right off of the drip tray, yet the bottom grate is left free.
BUT, I do cover the part of the bottom grate directly above the AMNPS with some foil to act as a drip shield, so that part of the bottom grate can't be used. I should build something else for a drip shield over the AMNPS that doesn't monopolize the left side of the bottom grate.
You can see the foil drip shield in this picture:
The generation 2 MESs are laid out differently, and from what I've read, Todd suggests taking the water dish out and putting the AMNPS right over the hole in that drip pan that was occupied by the water dish. That gets good airflow up through the AMNPS.
Which version of the MES are you using?