I think it's related to https://mojang.atlassian.net/browse/MC-6228
If the container have space for the item at the second slot of the hopper, the hopper don't move the item.
You can see in the screenshots. At the chest, there's space for the red wool.
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Well, I think that if it's possible to put the second, it should...
It's the same idea when taking the items from a container. If don't have space for the first item of the container, try the second. As they fixed when the hoppers are taking the items, i tought this one should be the same way
It's hard to make filters as the hopper won't feed a dispenser/dropper with 2 diffrent items(it will only try to feed with the first)
Duplicate of MC-8193.
Works as intended, the Hopper must first dispose of it's first slot before moving on to the next.