IME, the key to breeding is a healthy pair. This is what lots of live/frozen food does.
Another factor (in my case) is aggressiveness. Stressed individuals will never breed. !!!There are times when healthy and aggressive females would rather fight than breed!!! But this is quite rare.
With those two factors in consideration, I usually mass feed and isolate reluctant pairs, exercise daily about 15 - 60 minutes (my bettas are used to flaring for long periods). But if yours aren't used to it, gradually add flaring time so they won't become stressed (they seem to use up a lot of energy to flare). Eventually they will begin to "flirt swim", though flared to the same sex. A few days later, I breed them. . . . OR if I want to "force" them, I flare for long periods 1 hour/+. If they're healthy, they should be in breeding mode in a day or two.
Often (not always) different environment also helps. Say, they're kept in bare tanks then plop them in a well aged heavily planted tank. Breeding instincts usually kicks in.