I didn't have the problem when only loading 14. Is there something I need to do?
Load 14 rounds...
Barring some factory defect, I would say that with the fully loaded magazine you may not be seating it well enough. The fact that the malfunction occurs after a magazine change supports that as well.
Standard clearance drill for a malfunction is to TAP, RACK, BANG (or READY depending on your training). This is because it is not uncommon for magazines to not seat securely. The TAP is to make sure the magazine is seated.
Contact S&W to find out if there are any recall issues. If not, go to the range and ensure your magazine is seated securely. Keep your hand position to make sure you're not putting pressure on magazine release. If you don't have any problems, then run some reloading drills. Make sure you really smack the magazine into the gun during your reloads, and see if you can induce the malfunction again. If you're obviously seating the magazine and can induce the problem, its time to call Smith & Wesson back.
Good Luck, and let us know how it turns out.