why does the background app switcher die?

[William Krause]

My Lumia 900 used to be able to switch from app to app by holding the back key. Now it simply won't do it, ever. I also can't view the start screen this way when in an app. This was probably the thing I liked best about having a Windows phone, and now it's just gone. Is there any way to get it back?

Yes, I have done a hard reset.

 

[William Krause]

OK, so it turns out it's the same-as-usual Microsoft/Nokia solution: GFY.