Have you double checked to make sure your males are really males? Silly question, I know, but always best to be sure. You could try putting in the shed of the other male in with them next time to try and encourage a lock, and I have heard of some people even putting another male in with the pair they want to breed (closely supervised and not for extended amounts of time). The shed seems safer, IMO. Another trick that works pretty well is to wait to pair them together until a good heavy rain or storm rolls through your area.