Might be too late for honey depending on the flow, but, you can try this. Take the queen excluder off until they draw out about 3 or 4 frames, not fill them, just start drawing them. Then put the excluder back in.
As far as why they aren't using it, could be a number of reasons, they are storing it below, the hive numbers might not be large enough for them to want to go up there to store it. Is there a dearth?