When a hopper is given an item by a dropper, it does not update comparators that output its fullness level. To recreate
1) place a dropper facing into a hopper
2) place a comparator facing away from the hopper
3) put an item in the dropper
4) active the dropper
5) check the comparator that is testing the hopper then cause a block update to it, it should then update to output a signal.