Stiff springs in new M&P and M&P Shield magazines is a common issue. One of my 10 round M&P 45 mags was so stiff that it was impossible to load the 10th round when it was new. The solution is to load as many rounds as the mag will take - push the mag into the gun - put one palm on the bottom of the mag put the other palm on top of the slide and work the mag about 1/4 inch back and forth against the slide a few times before you push it all the way in in to lock. Then let it sit for a few days. That should break in the mag spring so that it loads more easily.