You could also check that the catch on the tailgate hasn't closed. I don't know what method is used on a Merc but usually there is a hook that snaps round the catch on the car body. If the hook has closed it won't lock round the striker.
also check that there are no obstructions around the 'clasp' - i have had vehicles in for this many times only to find compost ,paper,random objects in the recess around the clasp stopping the boot/tailgate closing fully and locking. But it's probably one of the Nylon bits inside the lock snapped / jammed. open it up and take a look !