If a dropper or dispenser is powered via a block with redstone dust on top. See first attachment.
Some of them seems to stay "triggered" even though power is off.

The first time the redstone wire on top of the cobblestone is powered all droppers/dispensers will activate. (I tested both.) When powered again some of them will not activate.

When looking at the droppers with f3 nothings seems wrong until you try accessing the inventory of the dropper. Then it shows that the dropper is powered. See attachments 2 and 3

Updating the dropper by placing a block next to it usally fixes it, even though it sometimes takes a few attempts.