The firing pin on the M14 & M1A does not have a spring around it. I would try a different magazine. Also, you can remove the trigger group and see if the firing pin moves back and forth easily by reaching your finger up to the firing pin tail on the closed bolt and EMPTY chamber. Have all your failures to feed been just when the cartridge is on the left side? There has been a couple threads in the past couple of weeks with this issue. I have to do a little back tracking or simple search to find the link. What kind of magazine have you been using?