Sometimes the bees kind of get stuck in a box and seem resistant to move up or down to another box until the population jumps. I have had some hives that didn’t want to move down to the bottom deep too. They used it for a pollen store.
Sounds like RHBee has about the quickest solution. You can always try to move bees into the next box by moving some frames of brood up/down, but you said the brood was in mediums and you want to get them to deeps. Yeah you can move some mediums into a deep box, but that can get messy as they try to add comb to the bottom of the mediums.