It all depends on the female. Just like people, we men can't force a woman into it. All you can do is feed her well...plump her up & regular water changes. If you have a pair that spawned before, that does not mean they'll get along the next go around.
I've had females that spawned every 2 weeks and others over a month between spawns. Each female is different...some will spawn at a younger age than others. Just be patient.