It does happen some time with new shotguns.
What make is that +2 rd mag tube, looks like a wilson combat unit.
Try this.
Loosen off the mag tube 1/2 way,then put you finger or somthing longerinto the mag tube where the follower is.
Push the mag follower in and out and you use your other hand to tighten down the mag tube.
If you feel the follower shifting under you finger then thats happened is the spring has been wound up as its been installed.
I have tryed this trick many time and it has fixed the problem.
It could allso be the follower jammed against the face on the inside of the mag tube due to a spring thats to long.
bbb