After my chicken farm design didn't work in different worlds, I tested the design and other similar designs and found that the redstone mechanisms updated inconsistently by direction. In the screenshot the mechanism on the left fires out all items put in the bottom dropper, but the one on the right never fires the last item. I think this might be something to do with the update order of the bottom dropper and the redstone next to it that powers it. (Left in screenshot is south or +z, right is north or -z).

Soory if this has already been reported but I searched about redstone update order and it came up with nothing.