Magazine springs can become weakened and worn over time. The company recommends that its LE armorers replace the mag & recoil springs either every 5K rounds fired, or every 5 years of use (since LE users typically leave their magazines loaded all the time).
So, how long have you owned your M&P40, and how old are the mags?
Also, there have been a couple of revisions of the .40 followers since the model was introduced in Jan '06.
Here are the .40 followers, running from the original/oldest on the left, up to the current/newest on the right (with the number 40 on the top). Notice how they've changed the profile to keep the last round better centered, as well as lifted higher? The front/left edge is higher, as well, (which might help with slide stop lever tab engagement when the mag runs empty).
This picture shows the oldest on the bottom and the newest on the top. Notice how the follower's legs/skirts have been changed, providing better lift and stability (under recoil).
If it were mine, I'd replace the mag springs and see if normal function was restored. If the .40 followers aren't the current ones, the company would probably mail you new ones for the asking.
Just my thoughts.