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I created two item sorters to demonstrate this problem. One has hoppers directly underneath the water stream, like so:

Key: I = Ice, H(arrow) = Hopper facing in arrow's direction, H = Down Hopper, W = running water stream
Bottom layer:

I I I H↓ H↓ I I

Top layer:
W W W W W W W

The hoppers are connected to an item sorter that has a comparator on the arrow's head's side. This sorter works.

The second design is the same, except the bottom layer is like this:

I I I I I I I
      H↓H↓

A chest is at a corner, causing items to be put on the ice and a hopper edge. The items DO get picked up by the hopper, but they do not get sorted. The dummy items also get sucked away as well. What is going on, and how do I fix this?

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    It's nice that you're including a text-only version, it respects the fact that images can become invalid. I'm more of a visual person though, so if you provide screenshots, that would be great too! It will also let us see more context of your setup. – ExpertCoder14 Sep 11 '20 at 23:13
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    The position of the hoppers relative to the ice is not the issue. They still need to be directed sideways, and the construction of the redstone part must be the same. My guesses are: 1. you directed them downwards instead of sideways, 2. you forgot the redstone torches to lock the hoppers underneath, 3. there's something powering up the redstone dust from a random side, 4. you used a non-overflow-proof design and one of the outputs is completely full, the filter hopper filled up and releasing filter items from its neighbors. – SF. Sep 14 '20 at 13:37
  • @SF. Yes, you are right! :D I realized that the hoppers are facing down, not toward the comparator. Is that the issue? You can put this in an answer, thanks. – fasterthanlight Sep 14 '20 at 13:47
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Any hopper has the abilities to:

  1. pull items from above, from any container or open space
  2. push them out into any container its output points into.

Locking a hopper, by powering it up, blocks both abilities simultaneously - it acts as an inert container - which means other, non-blocked hoppers can still insert items into it, or suck them out.

The sorter requires to pull the items from the 'stream' above, and simultaneously keep it locked in absence of the 'excess' items to keep the filter items in. This is achieved by using two hoppers: the top one holds the filter items and can pull items from the stream as long as it has room to grab them, and signal 'excess', while the bottom one is responsible for pulling the excess out, and not pulling any when there's just the "normal" amount left. That means the bottom hopper's sucking ability is locked out by a redstone torch in absence of sufficiently strong signal "from above" (its pushing functionality is locked too, which is an undesired though mostly harmless side effect). The top hopper's sucking ability is only restricted by the capacity, but we still need to cripple its pushing ability - otherwise the fact the bottom hopper is locked will be moot, as it can still have items pushed into, like any inert container could. In order to prevent that, the output of the top ('filter') hopper must be directed into any bogus direction where there's no container to push items into - for example, into the comparator. That way if the bottom hopper is locked, the items stay in the top hopper and aren't moved anywhere.

Failure to point the top hopper in a "wrong" direction is one of most common mistakes when building sorters.

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  • Are you sure that "Locking a hopper, by powering it up, blocks both abilities simultaneously - it acts as an inert container - which means other, non-blocked hoppers can still insert items into it, or suck them out." is true? I thought that unlocked hoppers cannot suck items out of a locked hopper. – fasterthanlight Sep 14 '20 at 14:28
  • Also, what about the hoppers below the top hopper? Do they have to be oriented toward the comparators and stuff, or can they just be pointing down? – fasterthanlight Sep 14 '20 at 14:30
  • @fasterthanlight It's true. In case of an overflow-proof sorter, the torch is located in such a way that it locks an additional hopper underneath the one normally locked if you place one there (the sorter's hopper is locked by power from the block powered by the torch, the one underneath by the torch emitting power to the side.) But test a configuration of a hopper with a redstone block on top or to the side, and another underneath, to see the the effect in action. – SF. Sep 14 '20 at 14:32
  • @fasterthanlight The direction of the hopper underneath only matters in regards of where you want to send the items. It's already the end of the filter, output it wherever you want. Just note any hopper under it will still get powered by the torch. – SF. Sep 14 '20 at 14:33
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    Pointing the bottom hopper into the torch is quite pointless as it necessitates more hoppers which you could skip totally. Consider chest chest ←H ←torch bl. ←rep. bl. – SF. Sep 14 '20 at 14:39

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