trunk locking problem
 

1999 9-3. My trunk closes and latches, but will not lock. If I lock the car, I can open the trunk by pulling on the handle, but then the alarm goes off. I want my trunk to be secure, but I don't know how to fix this. Sometimes I can hear the automatic locking feature of the trunk lock and unlock, like it's unable to stay locked. It does stay shut, just not locked. Any ideas?