Typically the tracks will rust out.
Also there tends to get a wear spot from stepping on them over the years.
Check all of the allen screws that hold the unit together. The screws will sometimes back out a little and cause the steps to bind up.
If a screw is missing, this too can cause the steps to bind.
you can check this link to see if you missing something:
http://www.airforums.com/forum...ep+replacement
For more info on steps and repairs, just type in steps in the search line.
Smily