Yes, they can wear out! The plastic shank that fits around the needle bar can break with extended use. A metal shank is stronger. I am not sure how the "innards" work, but the feed dog part of the foot should move when you move the fork up and down. Also, look to see that everything is lined up straight -- the toes, the shank, the feed dog, the box -- if any of that gets off kilter, you may be have the "even feed" action, but you will be fighting to keep sewing straight. On those expensive penguin walking feet for Featherweights, there is a spring inside the box that can become detached and just needs to be hooked up again.
I have had all of these things happen.
To make walking feet last longer, a person should not sew too fast with them. I have also been told that it is hard on the mechanism of a walking foot to backstitch. I imagine all of this depends on the model of the foot, since all walking feet are not created equal.
Dayle