Unfortunately, from what I know about all animals is that if they have tasted the best (in your case, brine shrimp), they won't eat anything else. I tried giving my betta flakes, and he just turned up his nose! I guess fish are kind of picky. By all means research more, you might find a way after all.

Sometimes Angels are picky, but they'll eat before starving. They will eat Flake food but the Brine Shrimp is a good choice. I would just keep giving them Brine Shrimp. A good option is one of those frozen multi-packs with brine shrimp, bloodworms etc. Some people cultivate mosquito larve and feed them finely chopped earthworms.