Try freeze dried blood worms perhaps? They look like they're "wiggling" when the current moves them, which really attracts the betta, because it looks alive. I found he ignores the pellets for a minute or two then eats them, but the blood worms he eats right away, whereas when he was in still water, he let both foods sit for a minute or so.
Note, freeze dried foods should not make up the base of their diet, as they are meant as "treats" or occasional meals, because they aren't "complete" like pellets. Feed a variety of foods though - thats the best way to make sure they don't get bored with their food, and also to ensure they're getting all their nutrients :)