Going through walls. Collision Problems

Ok, i am working on a platformergame, similar to the mario games, just differnt sprites. My problem is when the player falls down from his starting point he lands on the ground I placed, which is supposed to happen, but then when the ground goes up one step (like any other platformer) he walks through it instead of colliding with it. I am using hitTest for the collisions it just wont work. This video i was watching had the same problem, so if you dont understand my problem in text maybe teh video will help.

Video : http://www.youtube.com/watch?v=XK_63...om=PL&index=22

The problem occurs about 3:10 into the vid.

Thanks for any help.