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Stress, feed and light are probably the main things that could cause this - that is, if you are sure you actually have females. I had a female hatch around June last year and she didn't start laying till about 2 months ago. I think there was too little light by the time she matured. But if have 12 hours of light a day, and you are sure they are females, then you might want to elaborate on how and where you keep them, what you feed them and such..