My betta's favourite is frozen bloodworm and daphnia (not so keen on anything dried) which he gets once or twice a week, and he gets 2 or 3 good quality betta pellets daily. Won't touch flake or brine shrimp. Must be doing something right as he's pretty ancient now.
I've noticed both my betta and my gourami COULDN'T eat brine shrimp. They would both try and suck it in, then spit it right back out two or three times before giving up. They also both had trouble with flakes unless they were wet enough to start sinking. So to the OP, if you're using flakes for a bit, maybe just try pushing them under the surface with your finger?
I wonder if there isn't something about the mouths of those fish types that makes some food harder to swallow.