I’ve decided to replace the piston-powered “item slider” on the bottom of the mob grinder with a hopper attached to a chest. A similar setup elsewhere works fine, but here – the hopper accepts the items but doesn’t deposit them in the chest. The chest can open just fine, and in effect I have no clue what’s wrong that the hopper doesn’t deposit its items.
To me, it looks like the hopper is trying to deposit the items into the stone slabs next to it, rather than the chest.
If you place the chest, then hold shift and right-click the chest while holding the hopper the hopper should “snap” to the chest, like in my picture below on the left (ignore the gap).
If I put stuff in the hopper on the left, the items get pulled through to the chest.
The one on the right is what I think is happening to yours, the hopper is linked to the wall and isn’t passing items to the chest.