If you replicate what i built in this picture, you will see that the piston wont extand at all - the dropper it should get it's signal from is skipped (im pretty sure, since i can't see the item either).
Linked issues
duplicates 1
Attachments
Comments 2
I think this is a duplicate of MC-8457.
From my testing, the items do pass through the hopper, but the transition is too fast for the comparator to catch up. A workaround is to "prime" the center hopper by placing an item in it while the items are flowing. This will allow the comparator to activate and will stay active until the chest and upper hopper are empty. Unfortunately, adding new items to the chest will require another "priming" cycle.
To see the movement through the upper hopper, place the "primer" item in the last slot on the right and you will see the items from the chest showing up on the left.