I just bought my full size 9mm, (first gun purchase) and have been having this exact problem so I googled it, which led me here. Thinking I'm just going to send my 9mm FS in to S&W. My gun field stripped fine 3 maybe 4 times, then started messing up. Now it's every time. It's gotten worse and the only thing that has been helping was to pull the trigger.
I drop the mag
lock the slide back,
flip the TDL
drop the sear deactivation lever
then pull the slide back after which it should slide right off the front
...but it wouldn't, it would just lock in it's normal position
but pull the trigger and it shoots right off
this morning the slide slid off so fast that it slipped right through my fingers and the rear sight nailed the bathroom floor
which put a hole in my linoleum and scuffed up the top corner of my sight.
I know my way around a toolbox and I'm sure I can fix the lever nub myself but I'm hoping if I explain this to S&W they'll fix my sights too...
Since reading this thread I've tried pinching the barrel and guide rod together, which works great, but is however awkward.

Has anyone had the lever flip back up too early when reassembling? This happened to me which left the guide rod sticking out about an inch and a half and completely locked the whole gun up. the slide would move but the guide rod was stuck. I ended up screwing with it enough to get it back right, but i thought I had really screwed it up.