Probably, Wallpaper Wizard 2 is on pause. It could’ve stopped rolling pictures if you’ve set some particular picture as a permanent wallpaper (clicked Set Desktop Wallpaper). Or, maybe you just have no pictures in the Roll (one picture is not enough too).
Read here about how to add pictures and make them roll on your display.
To make a desktop background change again, choose Wallpaper Wizard 2 Menu > Play Roll.