It takes good strong nectar flows. You can help get them using the super added by moving a frame or two of drawn comb up into the super when adding it. For the first super over the broodnest, it helps to have frames of eggs and younger open larva in the box right under the first super added. Any honey frames in the broodnest should be moved to the lower box, that is if you are using more than one box for broodnest area.
They don't like honey under the brood, so they tend to move that honey up to over the brood. They store any nectar coming in that is more than needed for raising brood, up over the brood. Both of these activities will help to get them drawing wax up over the brood more quickly.