I have a plumbing system for fresh and filthy water, with multiple things connected to both circuits (a lavatory, a shower, a water purifyer, an air scrubber). Additionally, the freshwater circuit has both a pump, switched on and submerged, and a vent. Though water should in principle always be flowing somewhere, it isn't.
I've attached a screenshot showing the problem (water doesn't move), and a save file in which it is present. Hope this helps