I did make another PVC-tailed Plopper, but with an even wider and deeper tail fin.
Guess what? It turned when I shot air past it, but not in the water.
I put it next to my successful Plopper tail, and the original's tail, and I finally saw what was wrong.
I had made the end of the fin so wide it transitioned back into the body of the tail past the shaft, or axis of rotation, so it was fighting itself when I dragged it through the water.
I removed some material to get the connection to terminate even with the shaft, and reduced the size of the end of the fin and, plop, plop, fizz, fizz, oh what a relief it is, because it swims now.
I had avoided this problem by dumb luck with my first tail because I tried so hard to duplicate the original, I
Not understanding how the fin worked, I just figured bigger is better.
Lesson learned (I hope).