comparator doesn't work properly

The compartor does not give a redstone output when place against an inventory block like dropper or shulker box, it only gives a signal when the any block around it is changed/updated.
Similarly when the dropper etc. is emptied it will continue it give a redstone output unless it is given a block update. This affects shulker boxes, droppers, dispensers, chests, hoppers, minecart withe chest/hopper.