Fish Box set up

[The Kook Abides]

My Tiara 30 Open has a fish box macerator pump that operates with a switch. I can hear the pump switching on when I toggle the switch but it doesn't draw the water out of the fish box.

Do need to be underway/engines running? Did I miss a valve to open? Airlock in the plumbing?

Boat is new to me. It's a 2006.

Thanks in advance.