interesting , I have the 400HE with same grates and the only issue i have is the coals tend to block the ports in the back if you don't keep them raked to the front and away from those 2 flue ports , I usually have 3- 6 inches of coals and make sure i spread them out and away from the 2 ports in the back , only thing falls through is a fine ash in the bottom , no chunks
I think it is more on how it is loaded , i had to adjust my log cuts so they would be 30-40 inches instead of normal 18 inches , I also make sure i don't put big rounds in there but i split everything and pack it in there tight , when i put the rounds in there there is too much open space between them and they tend to bridge , I burn all kinds of wood , green , seasoned ,
timing also probably helps I tend to load mine when its half down instead of waiting until its completely down to reload , i average about every 6 hours when its cold , but i have 2 large greenhouses I'm heating and they can run through the wood when its cold ,