take into main dealer as it will be under warranty for this,even if its an import it still has a years warranty.
could be a badly adjusted boot hinge or ram.
would take it in asap before water gets into electrics which could cause major probs later on.
don't let them fob you off,by saying they can't find anything wrong etc etc
I would agree that this should be dealt with under warranty.
It might be worthwhile noting that when people have had similar problems on older MB models, it turned out that the cause was often traced to the rubber seal on the boot lock that had failed.