When you loot all on stacks of mats remaining in a furnace, large furnace, or refinery before they have processed, the ores /crude will often not stack. (i.e. if you take 200 ore out of a furnace that has been running, it won't stack with other ores in your inventory and has to placed back in the furnace as a stack of 200 and processed before adding more).
Server: Elysium