My little guy did that for the longest time! Then finally he went up the stairs, but wouldn't go down them! That went on for awhile, so finally one day I put him at the top of the stairs and coaxed him down each step. It took awhile and I used treats and sometimes I would place him on each step and then put him back and see if he would do it. Finally he did it! It just took awhile, but know he flies up & down them so fast that he'll miss a step every once in awhile and roll down the rest!
Its kinda funny. I make sure he's not hurt and then laugh at him! Usually he'll get right back up and try again!