Guys... I see most folk jump directly to equipment error...

Not to insult anyone, but have you all entirely eliminated the possibility of operator error?

I see one reference was made to 'limp wristing', and correcting this solved the problem. Next thing I would look at is how you shoot. Thumbs up or thumbs down?

If you shoot thumbs up you may very well be preventing the slide lock from functioning. The Sigma (and the SD) have honking great slide lock levers. ANY pressure on them will prevent them from activating.