There is a little piece inside the gas block that the large set screw holds in place. The bottom of that little piece has a hole in it that aligns with the gas port in the barrel/gas block. If that screw backs out the piece it holds inside will also move forward and cause the holes to become misaligned. This will restrict and/or completely shut the gas flow off to the piston, which would be the cause of the failures you started experiencing.
I have never had an issue with mine backing out so I won't advise whether or not a bit of loctite would be a good/bad idea.
I would pretty much suspect that the problem was that the set screw simply wasn't very tight when installed at the factory. I gave mine a good going over before using and most everything on it was not very tight.
Try tightening it back down good and tight and give it another whirl before you decide to get the loctite out.