did you add your box above or below? if there is honey above the brood, the queen may not cross it to lay. if there is no bridge between the bottom and top box, the bees may not move up.
the easiest thing to do is to move some of the stuff from the full box into the empty box. keep the brood together. when you move brood up be sure that you place it over brood below. put empty frames on either side of remaining brood in full box.
here is my observation which you may take or leave
in spring, bees build down in anticipation of the flow. they will store above....that's why we honey super the hives above. knowing this we still insist on putting brood boxes on the top and making the bees build up. while most hives figure it out and do ok, some don't.
it's kind of a pain to check the brood boxes that we have put under, but here is what i do: when i pull frames from the top box, i use a flashlight to look down and see what the bottom box is doing. there is no need to pull the top box off and rummage around in the bottom unless you suspect a problem.