Sometimes, adult ball pairs do not lock right away by just putting them together. But in these cases, it's usually the male who is not interested. Ball python courtship is male initiated, the females play a less active role. I have yet to see an adult female that refused a virile male, but i've seen a few passive males that do not show interest in courtship even when paired with proven females. There's a lot written on stimulating these males into breeding mode.