inseters sometimes get stuck trying to put fuel into things that have another type of fuel in them
Ex: trying to put wood into a machine that currently has coal in its fuel slot.
when this happens it gets stuck because it cannot put in the wood because coal is already there and the coal cannot be used because the inserter is not putting in the ingredient to make the machine go
Also, do not know if it is relevant but I am using the mod good burner things to add burner assemblers