They are notorious for getting very dirty and failing to function. Download and read this manual:
Field Service Manual
Section III part 2 deals with the trigger guard assembly.
Brief synopsis: remove the butt stock lock screw and butt stock screw, then trigger assembly lock screws and the two trigger assembly screws and drop the trigger assembly out. If the butt stock screw will not budge, remove the two trigger assembly screws and tap them out, this will release and tension on the butt stock screw allowing you to remover it(this is where most people break a cheap set of screwdrivers, especially if this screw has been in there for decades) Release the mainspring and then remove the mainspring lock screw and screw, and then dissemble, preferably on the kitchen table on a soft white towel, thoroughly scrub and put it back together, also that detent ball is like 3/32"
The manual covers it all in depth.(print two copies, one for the library and one for the bench.)