That's what I figured also. Field stripped everything twice and the rifle still wouldn't cycle. Everything else checked out, and was assembled properly. It was only after I picked the "gunk" out of the gas-port that it ran again.
Trust me, I'm quite suprised that the pressure didn't blow out any obstruction. But the fact that the M1 Garands port also seemed to be a bit plugged is what prompted the posting. Never had the issue before, and the only thing that has changed in my cleaning process is using foaming bore cleaner. It may be that I use a pull thru bore-snake, it could be a 1-in-100 occurence, who knows??
Just thought I'd post the experience to let others know the possibility.
NRA Life Member, SAF contributor.