I just bought a used S&W 910 today. Took it apart to clean, and I can't seem to reassemble it.

I've got the barrel, recoil spring, and guide rod back in the slide. I put the slide back on the frame (pushing down three pins in the back of the frame that stick up), but it won't move far emough back to re-insert the slide stop. No mater how much force I use, the slide won't move back from a normal "in battery" position.

I've tried following this video, but I'm stuck at 1:56. The pins are down, but the slide won't move any farther back than this:


Any suggestions?

Serial number is VCU2### if that matters.

EDIT: With the slide back on the frame (as in the picture above), I can't rotate the safety/decocker onto the "safe" position. If the slide is not on the frame, the safety rotates normally.