A walking foot is installed differently than any other Bernina feet. Remember to put the metal "arm" on the needle bar screw.
If that's not the problem, there's a little spring inside the walking foot that can break. Your Bernina tech should be able to open the foot and look inside to see if the spring is damaged or broken. I'm not sure if they can still purchase the spring by itself, or if they will make you replace the whole foot. They're very expensive!