it sounds like you may need a new locking mechanism. As in the part on the actual car, not the bar on the boot. They are fairlt easy to change, and you should ba able to find one at a breakers yard somewhere. It sounds like it may be sticking, so even soaking it in WD40 for a while may promt it to work again. They are however linked to an electrical solonoid. if the other suggestions do not work, it may be wise to have someone look at that.
Hope this in some small way helps ..
Johnathan Bailey
January 2005