It's a 6-rounder. Recently a few times the slide locked back with 1 round still left in the mag.

I compared the follower to my other 7 rounder mag and they look identical and in great condition. What could be the problem/solution? New mag spring? Thanks!

First, call Walther. They may do a warranty replacement of the magazine.

Do you shoot with the six rounder regularly? I ask because there is not a lot of grip real estate and maybe you are actuating the slide lock. That’s my Occam’s razor thought.

Other then that like para said try to get a replacement mag.

I don’t have that issue with my 6 rounders? Mag out of spec? Para’s right. Contact Walther or discard the mag.

Yup thank you folks. I will contact Walther Monday.

I don't shoot the 6-rounder as much as the 7 and 8-rounders but never had this issue before until the last 2 outings. Yeah could be me inadvertently hitting the slide lock but never happened before.