Hi,
usually molly give birth roughly every 6 weeks, but they can hold on for lots of reasons,
new fish added, being harassed by others, not feeling well, ect we have a platy huge with fry but shes never let them go, we have no idea why, all you can do is make sure the water is spot on, give her places to hide give a good diet in greens and brine shrimp / bloodworm ect and hope for the best. Donna x