Some droppers in a row fire only once upon their first powering, and refuse to fire again unless a block update occurs next to them.

Droppers in a row activated by redstone on a block above them all fire an item upon their first powering (provided they're filled). However, subsequent powerings do not trigger some droppers, which will never fire another item unless a block update occurs directly adjacent to it. After the block update has occurred, the afflicted droppers will again fire once upon their next powering before becoming unresponsive again until another adjacent block update occurs.