Man, what an ordeal. Using a mag in the gun really caused the slide to hang up on the way back so gave up on that. Always have been aware that the recoil spring should be in place and perfectly lined up so tried rotating the flat spring till the open end was pointing up. Finally got the slide lock to work. Whew!
May be a day or two before I try that again.
Should I still contact S&W for a new spring or does the fact I finally got it to work take care of that idea?
There have been a lot of complaints about new Shields being difficult lock back. I know because I was one of the complainers. (See http://smith-wessonforum.com/smith-wesson-m-p-pistols/432397-shield-rsa-one-more-time.html)
There is a very fun way to solve this problem. Go to the range a few times and fire the heck out of your Shield. Somewhere around 500-700 rounds, my Shield became as easy to rack as my other semi-autos. You can ask S&W for a new RSA, and it's good to have a spare, but using your Shield is the best way to fix your problem.