There was another thread on this and I set Bing as my lock screen wallpaper yesterday. It changed sometime this morning just like its supposed to. I have a 920 also. Maybe try setting it to a different wallpaper for while then set it back and see if that helps.
Im going to leave mine on Bing for a couple days and see if it stops updating.
Yeah mine is set to bing and it works daily just fine. Like suggested, maybe go to a static photo for a day then switch it back to bing and see if it starts working. Or maybe just restart the phone and see if that updates the background processes.
Mine just started update today. It didnt do that until yesterday and I got my phone last friday. Today morning, I woke up and wallah I had bing screensaver update by itself.