The Vacuflush holding tank has a tank watch level sensor assembly. The one on my 2000 380DA has three level switches, one for empty, one for full and one for half full.
Those level switches have sliding floats to trigger the switch.
Those sliding floats accumulate crud and stick.
On my boat I periodically clean the tank (after pump out), vacuum generator and those switches. That switch assembly has a 4" threaded fitting to screw into the tank, permitting removal for cleaning.
Follow normal diagnostics to determine / correct the fault.
Does the system have power? Does the vacuum generator run? Does a visual examination of the holding tank show it to be full, empty or partial?