When you have a supply line of hoppers, in my case filled with water bottles for automated brewing, the hoppers will randomly get stuck and not refill the stand with bottles. I tested it quite often and there doesn't seem to be a pattern. Sometimes the hopper feeding directly into the stand blocks, sometimes the hopper feeding into said hopper blocks. Forcing block updates with redstone clocks didnt help, neither did placing blocks around the hoppers. The only thing that helped was taking out a bottle. But even then sometimes it just forwarded a single bottle and then stopped.