Did you try it without the +2?
With the magazine cap off, and the spring just stuck in the tube, how much spring sticks out? (and if you're using a +2, factor that in.
I've always used Nordic's suggested method...
"On semi-automatic shotguns especially, magazine spring length can be critical for proper function. We recommend that approximately 16” extends past the end of the Nordic Components Magazine Tube Extension as a starting point."