We changed the difficulty from normal to peaceful and then from peaceful to hard, when we did this hostile mobs like zombie, creeper,spider etc, won't spawn even if we leave and enter game it only spawn some mobs for a short time, then it don't spawn any hostile mob. We even made some copies of the world, but it haven't worked, the same thing happened, hostile mobs spawn for a short and then won't spawn anymore.
Hope you fix it
thanks