When supplying power to a dropper, items will be "dropped" by only 3 droppers in a row, usually getting stuck in the 4th dropper. It has been noted that even with varied signal strength, such as a Redstone Block or redstone torch, and a really short line to the droppers through use of a vertical redstone clock (1 redstone repeater, 1 redstone torch, a lever affecting the redstone torch (to stop the clicking noise of empty droppers after use as it is no longer being powered) and an output line to the dropper, which should allow for full signal strength, that not enough of the droppers are receiving power..