Heatmor works like this, if there is no call for heat fan is off, when you open the outer most door (not the firebox door) a switch turns the fan on, I believe this is to give some oxygen to the firebox to make flashback minimal. Then you turn a normal looking light switch off which turns the light on and fan off.
kso: I am new to OWB's and the heatmor as I have been burning mine for only a month and half now. I wasn't too impressed with mine in the warmer months...40-59 degree weather. I never did mess with the differential, wish I would have. Fire went out one time for me and I think it was from a low demand for heat on a warm day and burning round birch thats a bit punky, but the main issue I had was the fan flapper would either stick open a bit or closed from creosote building up on it.
I'm guessing you don't have enough load on the boiler but its possible maybe a fan flapper stuck closed and couldn't fan the flame. I think your 400 has two fans, one in the back and one in the front. Might be a good idea to take both out and see if sticking closed was a problem.
I've also noticed that the water temp on the gauge on the front of boiler will read 10 degrees plus or minus a couple colder than what the aquastat is set at. So if aquastat set at 180, which is all the higher my heatmor goes to the gauge will be 170ish when the fan shuts off.