I live on a the 3rd floor in an apartment...Kelsey runs up and down, she loves them. I think they give her good exercise. She actually "races" me to the top and downstairs to go potty...she wears me out. By the time we are both up the 3rd flight we are pooped and ready for some yummy cold water! lol I think it just depends on the dogs personality. At first Kelsey looked down in fear and wouldn't go down them but just up...and now she does both. Maybe your baby isn't used to them..or is just lazy and doesn't prefer to go down them.
