Disassemble and rotate the recoil spring a little until you hit the sweet spot (you will know because it will be easy to lock back when you re-assemble). I had this same issue. On January 14, 2015, I called S&W and described the issue very nicely to a CS rep. I offered to pay, but the CS rep would not hear of it. The new recoil spring assembly arrived today. The packing slip showed it was "picked" from the parts department on January 20.
Given all of the folks who wait just as long for an answer to an email, I AGAIN strongly suggest that anyone with an issue simply call and speak nicely to a CS rep.
In over 40 years of calling S&W, I have never been treated rudely, nor have I waited months for parts, etc.
Give it a try. S&W has a spotless record with me.
To those of you who might suggest that a pistol should not make it out the door with a bad spring, the score is now 1 - 1, so S&W and Glock are in a tie for each having to replace a spring on a brand new pistol received direct from a distributor.
Good show S&W!