Run a dowel through the center of the spring and compress it (not inside the extension though) and measure the total length of the compressed spring. If the length is much longer than the distance from the groove to the closed end of the rod cap the spring is the problem. If it is shorter, the follower may be the problem.
To verify the follower is the problem, remove the follower from the extension and reattach the extension to the magazine. Place the follower and 7 shells next to the shotgun (follower next to the groove) and check the total length of the follower and shells to see if they should fit.
If it looks like they should fit, then you have another problem.
By the way, are you absolutely sure you're using 2 3/4" shells? :wink: