Black Tanks Won’t Flush

I have no idea if this will help because the OP hasn't provided any clarification. The black tank flush in my fifth wheel quit working, as in water would not flow. I unscrewed the nozzle from the tank to inspect it. It has a screen on the end & it was completely clogged with fibrous material. I cut the screen off & soldered on a short length of copper tubing with the end pinched a bit & reinstalled it.

The flush nozzle now has its discharge aimed in such a fashion that it causes a circular flow in the tank. It really flows a lot of water now.