this isn't a common issue with these clamshells, but I agree it usually happens from a drop.
Not sure the exact epoxy compound apple uses but we haven't found any that will properly hold it in place once its happened. Sadly, your best bet it to get a new clamshell. If there is no sign of physical damage, go to apple and speak to a manager. If they don't do it, call apple and ask for customer care.
If you have to purchase it on your own, it's pretty pricey.