Yup, I have the exact same problem as the original poster with my recently purchased Shield. (I also had the yellow sear deactivation lever pulled down during reassembly.) And I was very, very careful to line up the recoil spring assembly both vertically and horizontally. To get the slide fully retracted I had to increase the force needed by about 3 or 4 times.

For my gun, the problem is definitely the recoil spring since the slide moves freely without the spring installed.