Totally isolate bettas that won't flare. If you "force" them, they will become more and more stressed and won't flare for a long time. After a few days of isolation, you can begin flaring him to a docile female for a minute or two. Add time each day until he will flare to females for 15 minutes. Then start flaring him to another male.
If he still doesn't respond, put a gap between their tanks and only open an inch of the bottom part. And place a female next to his tank. He should mainly flare against the female, but this should give him enough courage to eventually flare against other males. Just don't flare him for too long. Change the color of sparring partner every two days or so.