Hmmmm... I recall mine was kind of stiff when new, and I did fumble with the slide lock while reassembling the first time. I did get it locked back though. After a couple/few hundred rounds, the gun has loosened up nicely on all accounts.

If it locked back on the empty mag, and you were able to remove the mag and it stayed locked back, then the slide lock had to engage the slide.

I'd say a call to S&W is probably in order and will clear it up.