Hey Steve,
The moving/flopping fish sounds like a good idea. If it's sense of smell is impaired, then maybe the visual stimulus will help. Does the snake come over to investigate the smell?

Otherwise, the next time you're having steak for dinner, try cutting off a thin strip before you cook it. Then try introducing the meat to your snake. Put a bit of calcium powder on one end, and see if your snake will swallow that, like it does to the guppies. The meat is more nutritious, and will last in it's stomach a bit longer. It may help your snake grow faster, as well, without having to feed it as often. A bit of variation is always a good idea, too. Maybe when it's larger, it will eat better. It's also possible that due to the intense inbreeding among flames that you've just been given an unfit snake that'll take a lot of pampering.
Rick