Bill,
Besides a stoppage, the only way I can imagine the stand pipe would stop flowing is that the outflow end is under water that's at a higher elevation than the pond. The water would probably be nearly as high as your dam, and you would certainly know that.
If the outflow side is visible and well below your dam, you certainly have a stoppage. All it takes is a few twigs to start catching leaves, grass, and algae. That combination can make a very effective plug.
