A hopper minecart set up as a filter (41-1-1-1-1 items) will sometimes not pick up the filtered item from the on top of the block above when other items exist on the same block It appears to depend on the order they were placed.--

My test setup is shown in the "farm setup" image. The top minecart has a detector rail and the lower minecart has an activator rail to set up a filtering system. The items in the top minecart are distributed as shown in the filter setup image.

I place a block 2 above the farmland and drop kelp and another item (wheat in this case) down onto it.

What I expect to happen: The kelp is picked up by the top hopper minecart and filtered to the bottom one.

What actually happens:

(a) If the kelp was dropped first, it seems to work correctly

(b) if the kelp was dropped second the hopper minecart is unable to pick it up and it remains as an item in the block above the farmland.

(c) If kelp is dropped, followed by another item, followed by more kelp. then all the kelp will be ingested, as long as the second stack of kelp was placed before the first stack had finished ingesting