In the 1.1 changelog they fixed hoppers not pulling items from above if all of its slots were occupied. This seems to have broken something with dispensers. Now, a dispenser will not dispense into another one if the receiving dispenser has all of its slots occupied. To test, place two dispensers/droppers facing each other. Fill every slot of one with a single stackable item. Then place one of that same stackable item into the other dispenser/dropper. When you try to dispense an item into the slot-full one, nothing happens.