Well, assuming you put it right back where you found it at the end of the mag tube on the gun, there's your problem. Pull that spring retainer out. It's a tight fit in the end of the magazine tube and it's designed to keep the spring from shooting your eye out when you take the magazine cap off. If you re-install it in the end of the magazine tube you haven't changed the tube capacity and it doesn't have the ability to move forward into the additional capacity of the extension. Take the retainer out, put your spring back in the tube, screw the Nordic extension back on and you'll be good to go. Unless, that is, you cut the spring too short when you were making your adjustments. Cross that bridge when you come to it.