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.
- duplicates
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MC-8193 Hopper doesn't try to place items from other slots if unable from first.
- Resolved