If you haven't already tried, try lubing the latch itself. If that doesn't work remove the trim covering the area that contains the locking mechanism inside the trunk lid. See link, number 6, http://www.realoem.com/bmw/showparts.do?mo...810&hg=41&fg=60
This second link shows you how the trim panel (covering the locking mechanism) is atached to the trunk lid.
Same thing happened to my 318... the latch stayed open. I sprayed some silicon and then I used some oil. Took a few tries but it finally opened up.
Hope this helps. If it doesn't then you'll have to look at the central locking system but I doubt that is the problem.