If you're dealing with Comfort Colors, the only ones that will discharge (some do ok, some don't at all) are the reactive-dyed colors, which should be separated on your swatch card if you have one. They're a pretty thick and nappy texture to print on, so you'll have to do a bit of playing around to get your process down for them.
The full pigment dyed ones will basically laugh at you if you try to discharge them.