In my experiment, i had a hopper with 4 stacks of 64 charcoal and 1 stack of 63 charcoal. the hopper was attached to a comparator and i was checking signal strength. when I dropped 1 charcoal onto the hopper it worked as intended and picked it up, however, if I drop any other item onto the hopper so it is constantly being rejected, then I drop the charcoal while the other item is there as well, the charcoal will be rejected and never go into the hopper even with room for it.