ah okay your gas system sounds fine then, its either your trigger pack, or the bolt release button. you can see how the bolt release works pretty easy when the gun is disassembled and the trigger pack is removed, part of its job is to keep the round in the mag tube from getting onto the follower until the gun tells it to.
as stated above that button with the red dot on it forward of the trigger manually releases a round from the tube onto the elevator, and when you pull the trigger that button is pressed on its own by the mechanism and works with the bolt release assembly, there is a little wedge that catches the rim of the shell that is in the tube, well that wedge moves to the side and the shell pops out. something in all of that is not communicating properly to tell the shell in the tube to get on the elevator, and then the elevator to raise the shell up
the user B. staring on page 21 of this thread had a very similar problem, hopefully he can chime in if hes still monitoring this thread. at first i thought he was saying his gun wasn't firing, extracting and ejecting rounds but i think what he meant was rounds weren't feeding from the tube to the elevator and/or the elevator wasn't lifting the shells up
im guessing there is a weak/broken spring in there, or perhaps some gunk got in where its causing some issues, maybe something like a pin has broke, i really couldn't say without seeing it in person but it sounds like your issue isnt the gas system
tomorrow when you can tinker with it some more, try loading the tube up and putting a round directly into the chamber and see if it cycles properly as a semi auto should. if not, after firing a shot and spent casing is ejected, look to see if anything is on the elevator. if there is nothing on the elevator and no new round in the chamber then the wedge that controls the rounds in the tube might be getting stuck. if there is a round on the elevator but no round in the chamber then its your elevator that's the problem
we're getting closer to figuring out which part of the gun isn't working the way its supposed to