redstone dropper problem

I have 18 droppers all wired up the exact same way with redstone but for some reason 2 of them won't activate when i activate the redstone and even when i just put one redstone dust and a torch to activate it it still won't activate but when i break the redstone dust it activates. It only happens to these two droppers and doesn't happen to any of the others that are wired the same way i have checked all of them over and over again and i know its a bug and not a problem with the wiring.