Basically the hoppers can't get the items from above if all their slots have items in them. This breaks sorting systems because hoppers will be filled with items and waiting to receiving only certain items that will never will be pulled out. This works with any kind of situation including dropped items, chests and other inventorys (like hoppers).

On first pic I show the set-up:
Red tagged situations don't work correctly and the green is the only sutuation works correctly (because the top hopper actually put the item to the bottom one)
On the second pic I show what the bottom hoppers have
And on the third what the top hoppers have.

What should have happened is that the items on the top hopper need to be sucked up to the bottom one but that didn't happened.