My guess is the flush-nozzle has gotten clogged-up.
All of the black-tank flush systems I've seen have a vacuum-break valve plumbed into that line somewhere. It helps keep anything from back-washing into your fresh-water system when you shut off the tank-flusher by releasing any pressure/vacuum in that line when the waterflow stops.
However, it's sometimes common for bits of .... ahhh..." material"... to get sucked a little ways back into the tank-flush nozzle holes. Not far, but just enough to begin to create a little clog, and with continued usage it may get to the point where it gets completely stopped up.
I'd suggest first taking off the nozzle at the tank, and checking it for stoppage. I've successfully cleaned mine out and reused it, but some folks just replace em. They're inexpensive.
If that isn't the issue, at least you can see if you do get any waterflow at that outlet when you turn on the tank-flush system, and can then check backwards up the line. Next guess would be something stuck in the vacuum-break, or maybe the break-valve itself. Or perhaps the line is pinched/kinked somewhere?
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'06 HR Endeavor 40PAQ (Mission Hills decor). All options, but still finding more to add.