My opinions: the male and female shouldn't be kept together all the time...they should only be moved together for a brief period of time to spawn. The female should be removed if spawning does not occur.
What are you feeding? The female betta needs to have good nutrients and great water quality to produce eggs. (She will plump up.) Most people recommend tubifex worms.
Same for the male - do daily 20% water changes and don't introduce the female until he has already started making a bubble nest.
Where did you get your bettas? In my experience, large mature males are harder to breed than younger, smaller males.