ODN You are probably right about the spring but the part that bothers me is that you said the rifle seemed to be ok before diassembly.

A couple more possibilites:
Is the trigger a GI or an aftermarket (Timney etc.)?

During Reassembly di you tighten the guard Screws, especially the rear screw.? If the the screw is a little over length it might be interfering with the firing pin extension.

Regards,

Jim