this may be your problem: is your trunk's keyhole horizontal?
go to your trunk. take your master key and sitck it into the key hole. turn the key all the way to the left. (this may open the trunk)
from now, the key hole will stay horizontal.
check your owners manual. it says all of this in there. (it says, when you put your master key into the trunks keyhole and turn all the way to the left, this disconnects the trunk from the central locking system and from the outside: and only the master key can open it.