Character is walking on air?

Not sure if this is the correct area to post this.

Problem:
My character (PC) walks and goes up stairs (a box collider set at an angle).
But the problem is that the character doesn't seem to go down them. The character just carries on to walk straight on and walks on air (there is not a collider beneath him).

He has a character controller attached and a script for animation.

The characters collider is set fine, it's not too long or anything. I added a rigid body but it made no difference. Increased the mass, still no difference.

He walks up fine but doesn't want to go down.

Any ideas to why this is happening?

 

Is gravity activated?