In most cases, if you feed them very well (worms are recommended) and get them cold in winter, nature will take its course. There may be other ways to manipulate them, but the simplest is to use these natural cues of nature. Some people say that a large water change will get them going.
How old are you axies, and how long have you had them together? Right now in the southern hemisphere, it's sort of the wrong time of year to think about breeding.