Comarator not detect the signal of a powered block by redstone dust when dropper/dispenser is against this block.

1 Place a dispenser/dropper/hopper/chest
2 Place a block against dispenser/dropper/...
3 Place a comparator against block for compare dropper
4 Place a redstone dust on the block between coparator and dropper/dispenser/...
5 Power the dust with a torch or level

Comparator not detect the power.