Hopper stops hopping when dropper stops dropping

I am trying to build a iron sorting system where excess iron gets sent to a different location using water elevator. However, when the dropper stops because it reaches its activation limit, which is 1 stack and 17, the hopper going into the dropper also stops. Even though there is still space in the dropper.
When the dropper is dropping, the items in the hopper do go into the dropper. Does anyone know why this happens?

I added photos of the system, dropper and hopper inventory.

Thanks in advance.