Hi LaCrawfish,
Reading between the lines, I'm guessing that you open the dump valve and some water comes out with some waste material. Then the flow of water stops before the tank is empty.
You may simply have a blockage of paper and waste stuck at the valve. That can happen if you use a lot of paper, or use paper that doesn't degrade quickly, or if you don't use a lot of water when you flush.
It can also happen if you have a slow leak from the black tank valve, and have the sewer hose connected all the time. Most of the water leaks out of the black tank, leaving solids behind, piling up and getting progressively more difficult to dump.
When you insert the water hose, if after 20 or 30 seconds there's a rush of water and solids, like a dam burst, you probably have a buildup at the valve.
It's possible that manufacturing debris could be clogging the valve opening, but since water clears the problem, I think that's less likely.
You might take a look at pages 17-22 of our owner-written
Water Systems Guide for tips and best practices to dump the tanks.