Vacuum head 2000 340

Our vacuum flush is not picking up water
It sound like it is trying. I am assuming the flush draws water from the lake and not from the water holding tank. Also is there anything different that has to be done at pump out compared to a regular heads.

Vacuflush heads use the potable water supply on the boat (or your dockside water supply if connected). Have you got water supply for the taps at the sink?