This relates to MC-110236 and similar ones. These reports decribe the issue as a failure of hoppers to transfer items down from a hopper line (usually in furnace arrays), however the underlying cause seems to be different: Every 8th item in a continuous stream of items though a hopper line doesnt actually exist. This can be verfied quite easily: A long (e.g: 64, see attached pictures) hopper line of n hoppers with comperators next to each hopper is fed only 7/8*n items. For brief moment every comperator will light up, which should indicate that there is a item in every single one of the 64 hoppers. This is of course impossible as only 56 items were placed in the first hopper. Should hoppers below this hopper line be briefly unlocked the system will "realize" (for lack of a better term) that every 8th item doenst exist and will, correspondingly, and somewhat correctly, not pull the non-exiting item.

I did a test on a similar hopper line (bottom one, pulldown_test.png), every comperator was lit up at the moment the bottom hoppers were briefly unlocked by manually removing and placing the center redstone torch. Every 8th hopper did not pull down an item (empty bottom hoppers marked with a sandstone block next to the corresponding comperator).