Lavatories suddenly not accepting water

I forgot to add, the water is being pulled from a reservoir, and the waste is being dumped into a separate reservoir.

First, try to save/load the game. There are multiple issues with the game's piping system recently that get remedied by save/load.

Second, don't draw drain pipes through building outputs. When a dupe uses sink/lavatory in the upper floor, its output blocks the sink/lavatory in the bottom floor while the polluted water is traveling over its output. That may get a dupe expelled from the toilet before he did his business and may even make him pass the sink with germs still on him.