Droppers Receiving a Redstone Charge Won't Drop Items

Sometimes, when an upward facing dropper receives a restore charge, it will not move/drop the items inside it. As you can see in the screenshot below, there was an item placed in the dropper (inside the tower). The comparator is used to detect the item inside, and then power the dropper to move the item on upward. (I have used this method to design my own version of an item elevator). The only problem is, when the dropper was powered, the item did not move. As seen in the picture, the item is still stuck inside.