Could be a mag problem: mark your mags and see if problems occur specifically with one particular mag. I use a paint pen from a craft store to number my mags. If I notice a mag developing a trend for feed problems, it gets rebuilt or trashed.
Could be a recoil spring problem. A weak spring could prevent the proper feed of the next round.
Could be an ammo problem. You say you've fed it lots of different types of ammo, but do some types run better than others?
Could be a shooter related problem, such as grip interfering with the slide or controls, or limp writing. Pay attention to your grip and making sure you don't contact anything but grip and trigger. Also make sure your wrist is locked tight.
Could be a completely separate mechanical issue. If the above steps don't indicate something obvious, then you should definitely call S&W.