Black tank flush not working

A black tank flush system that doesn't seem to do anything is usually a clogged flush nozzle inside the tank (mineral buildup) or a failed check valve letting water flow the wrong direction instead of into the tank. Confirm you're actually getting water pressure at the flush inlet first — if there's no pressure there, it's upstream of the tank. If pressure's fine at the inlet but nothing happens inside, the nozzle or the valve is the likely culprit. Have you checked pressure at the inlet yet?