find a local smith. for as many different variants as there are, each has its own issues. some have mag problems, some have headspace problems, some have piston problems.. its best to be able to se it to try and diagnose it.
Great advice. A few years ago I bought an STG58 version of the FN FAL. It wouldn’t fire either. I took it to a good smith that fixed all the problems many of which you mentioned and it now runs like a champ.