[Eeyore]

It could be three things. Take out your bolt, look at the white plastic spacer. Where is it? It should be at the very end towards the stock. Sometimes it will move and cover two holes in the bolt carrier and this will happen.

Second issue could be if your selector switch is very warn or damaged. Take it out and inspect it.

Third issue is if part #48 and #54 and #57 are not seated correctly or if they are malformed or damaged.

My bet is your spacer.


Edit; Beaten to it by Ballcancer while typing.