Factorio Forums

Inserters will not continue placing objects/resources into assembling machines. Inserters will pick up 2-4 objects and place them into the machines and then suddenly stop. The machine needs more material but inserters do not pick up at all.

not a bug, the insters will place about enough for the assembling machine to create 2 items, then it waits for the assembling machine to use some of the items.This helps in an assembling lines to prevent the first 2 or 3 machines from taking all of the resources and leaving none for anything further down the line.

The inserters also stop when the output slot of the assembling machine is "too full" (probably >=recipe-output-count*2 ? )
This prevents the same situation as FreeER mentioned. (Items would stockpile in the output slot of the crafting machine and needlessly hog resources)