[Imogen]

There's a few reasons behind this so it depends on various factors.

1. It is either quite humid / hot
- so the hamster prefers sleeping more above substrate

2. It's their personality...
- My Syrian, Remy likes to burrow a fair bit in to the substrate in his nest. But my recent addition, coming from what I can see when I peep inside the door opening likes to sleep a little more above the substrate.
But your hamster may be one that just doesn't want to sleep there. There's a lot of hamsters that pick sleeping under or beside their wheel because they like it there. I've heard people mention that some of them love their wheel so much they make a nest on the wheel itself.

3. They're stressed because their environment is too small
- You mentioned the robo living in a small 2 story cage. Could be possible for your little guy since they require space and not height. Hams don't care for height. They require at least 4000cm2 (620 square inch) of unbroken floor space. You find common answers on here that state about 450 sq inch; but that's kind of outdated. People recommend 4000cm2 and for a female 5000cm2. My rule of thumb is for smaller species though that russians go for at least 5000cm2 since they are way more energetic and fast. Although you think they need smaller space due to their size - is the complete opposite. Bigger the better.