The cover , obviously , should be on there mate , Alot of second hand parts are available on here , from FBI , NOSVW etc etc
The cover helps keep the sliding hinge in place , the problem could also be the bottom runner on the door ,
A few pictures may help us help you too
http://www.ratwell.com/technical/SlidingDoor.html#adjust" onclick="window.open(this.href);return false;
Let us know how you get on 8)