It's normal for axolotls to gulp for air at the surface once in a while.. however, if it seems excessive, something may be up. Like Aviette said, have you checked the water parameters?
Also what type of substrate are you using, sand, gravel, rocks?
Is s/he eating and using the bathroom as normal? I've heard of axies floating and then using the bathroom/letting gas out and going back down to the bottom.