Dispenser won't dispense

I made a game where you click on creeper heads. https://i.ytimg.com/vi/vXtQau_Z8Og/hqdefault.jpg These four dispensers are each supposed to dispense a creeper head on the armor stand in front of it, but the fourth one doesn't, even though it's being powered in the same way as the others. Yes, there are heads in the dispenser. It also happens to be downloadable, so you can see it for yourself: https://www.dropbox.com/s/vk6okzyydgp7u3t/Whack-a-creeper.zip?dl=0