I flush EVERY time I dump the black. While you drive, things splash around and it's possible that things may have gotten stuck on the nozzle.
There is likely a check valve in the line. I hunted one down in a friends unit, it was in a utility void, it was a vacuum breaker/check valve combo. It was stuck shut after lack of use. You describe it going into a wall then back down - it likely goes up there because a vacuum breaker is up there.
1) If the coach is not level when you use the flush, it may take a while for water level to build up and start flowing out the clear elbow.
2) with a 3/4 full black tank, you can add some cleaning solution and drive around for a while in town. Turns and start/stops should give the top of the tank a bit of a clean.
3) Visit a dump station when it's not busy, connect flush water, and bust out a lawn chair. Leave the drain open, and just let it run for a while. See what happens.
4) Flush for a bit after every tank dump in the future.
5) Find a way to locate that loop in the wall. I doubt it's kinked, but there may be a vacuum break/anti-siphon check valve arrangement in there.
2003 Fleetwood Discovery 39L