Not being able to get the slide all the way back is usually the result of the recoil spring assembly (RSA) causing some kind of a hang up. Make sure that the RSA is in perfectly straight when looking at it from the top and from the side. Also, try rotating the RSA a little at a time until you find that "sweet spot" where it doesn't bind. I usually found that the area where the flat spring is cut (at the chamber end) had to be pointing up at me when looking down at it.

I ended up contacting S&W and told them about it. They sent me a new RSA and I didn't have any issues after that. I since traded that Shield 40 for a Shield 9 and the new one does not have that issue either.

You might be told various methods for locking the slide back or to shoot it a bunch but when the RSA binds the slide stops and won't move any further. It is the weak link in the design for sure. Good luck!