Sounds like what you are referring to are the living hinges that are between the side panels and the beds that allow them to slide as well as lay down on the bed. If they are intact and are just sliding along with the panel, them you should look at the channel on the bed that these slide in. There should be a small screw that holds the hinge from sliding on the outboard end of the bed. Make sure that this screw is there and doing it's job.
If the hinge is broken, you can get new hinge from Elmer. Go to the old site and click on Apache Parts & Service. The company he works for is the first one listed. The new hinge is made from canvas so that it won't get brittle and break like the plastic hinge.
