Why do pellets build up in the fire box of my pellet stove?

Pellets will build up for numerous reasons. First thing I would check is the air flow settings on your pellet stove. If it is too low not providing enough air the pellets will not burn fast enough causing them to pile up.

Another possibility is that the feed rate is set too high on your stove.

I would recommend cleaning out your stove really well, removing all the ash and trying to see if they pile up again.

You really have to mess with your feed rate and your air controls to get the proper settings where it runs smooth.