TM - Does it work okay with the other magazine? Just curious, if it works with the other, call Smith. Sounds like something is not right inside the magazine.
Other question (second thought), the magazine allows you to load the 6th round, but you are saying the pistol will not allow you to seat the loaded 6 round magazine. OR the magazine won't allow you to load the 6th round? Sorry, I didn't mean to confuse the issue, but I read your post again, and I then realized it might be more than my first suggestion!
Rick