Can you hear water flowing through the flush port? is the hose connected to the correct flush port? on our '24 44V14, both flush ports are in the wet bay. I think I have seen on newer models there is only the front black tank flush there, leading me to believe the rear black tank flush may be closer to the actual tank in the rear. Other than that, there should be a vacuum breaker that you can check but those leak more often then actually block flow. If you can access, try and follow the white PEX from the correct flush port and see if you can see any glaring concerns.
