I'm testing a theory now that the lower and right walls have to have something in the square behind them to show up, thus they can't be placed on the outside edge.
[edit]
That did it. For the lower and right sides you can't place a wall on the outer edge. It's just a hunch but I wonder if you were able to place the camera off the right edge and watch as the level starts up, if the walls would fall.
Anyway, that seems to be what is happening. If you don't place a ground tile for the wall to sit on, it wont show up on those two sides of the map, even though I actually placed the walls from the adjacent tile that already had a ground tile.
New map attached.
It just occured to me that if the wall could "fall through" because the adjacent tile didn't have anything in it, I wonder if that might have something to do with the occasional "stuck" problem.
[edit2] Ok, so maybe they didn't fall. It occured to me that they probably wouldn't be set up as physics objects affected by gravity..
Random weirdness is so much fun to track down..