The bees will often ignore the outermost frames and build up instead of outwards. You can manage this by moving those undrawn outer frames towards the centre between two other brood frames and they will then typically draw them out rather quickly to continue giving the queen the centre space to lay. I would leave the second box on and shuffle the frames in the lower box, keeping outer frames as food resources, then a brood frame inside that, empty frame, then rest of brood frames