this happenend on mine when i fitted two 6x9's. because of the extra weight, it's obvious that it's gonna be a strain on the shelf. when you open the boot, on the exterior, just below the screen, there is a little ledge/shelf, about 2inches wide.i found that the extra weight pushes the shelf down onto this ledge.i packed mine out with some thin "foam" "sponge" stuff i got from work.only about 2-3mm thick.anything will do, as long as it sticks where the shelf makes contact with the ledge, and they don't touch after.
hope it makes sense!!!!!!!!!!!!!