Could be maturity or the fact she just isn't interested. If the weights are right just keep pairing them up and then giving them down time. You may not see anything for a couple months. Also they may be doing something when you are not watching.
Addtionally for some reason I have seen that the females do not seem to be as interested in younger males. If your male is too young he may just not be mature enough himself or really have the instinct to stimulate the female properly yet.