I had exactly the same problem and found that the reason it works loose, is because the hex headed bolt that screws in from the back (which had fell into the trem cavity on yours) screws up against the trem arm and gets turned loose by the use of the arm.... I placed washers beneath the bolt of mine to overcome this and also allow my trem arm to swing out of the way when not in use. The only problem I have now is that there is play in the arm socket and so rattles slightly....I need to find a way of having the arm swing down but to be a better fit in the socket.
