The reason slides don't hold open on the last round can be traced to one of three reasons: 1. Defective slide stop (probably not your problem since it does activate, though incorrectly with the factory magazine) 2. Defective magazine spring ( easily fixed by replacing the spring which may be worn out or insufficiently strong) 3. Defective magazine follower (also possible but replacement followers may or may not be available for purchase new). Springs are easy to get, low cost and are good to have as spares. I would try that first before trying to find a new follower. I wouldn't replace the slide stop till I tried both of the other two.