I know this is going to sound stupid, but I do stupid things all the time.
Make sure you have males and females. You'd be surprised how many people think they have a mating pair, but in reality they have all the same sex.
Reduce stress for them. Like the rest of the people are saying here, provide clean, stable water for the fish. Feed them a variety of healthy foods, and give the plenty of hiding places.
Mollies naturally are baby making machines, so it's possible that conditions aren't ideal for them if they aren't producing. Most people have the opposite problem and tear out their hair with the overflow of fry. Good luck!