My robo hamster doesn't like tubes =(

I got my first hamster pet - Roborovski Dwarf Hamster. I did read as much as I could about them and so far grasped the whole "hard to tame" concept as well as the "pets for watching rather than to play with".

I love my hamster, he is awesome, but one thing does puzzle me is this: his cage, as pictured here http://i.imgur.com/I4LNm.jpg - is not exactly giant, but has 3 floors. I would like my hamster to utilize the entire space of the cage. The only way to access it is by either plastic ramp or plastic tubes (I replaced ramps with tubes). But he doesn't seem to go upstairs at all/ever. Even when I place the food up there, he still doesn't go up.

Is there anything I can do to motivate him to go upstairs on his own?

Thanks!