Changing wallpaper by scrolling on the indicator

Hi everybody,

One of the best tricks of Variety is changing wallpaper by scrolling on the indicator (tray icon), when scrolling down you get the next wallpaper, while scrolling up you get the previous one.

On Ubuntu 12.04, everything was ok, but with the other distibutions (12.10 - 13.04 - 13.10 & 14.04), what ever you scroll down or up, you always get the previous wallpaper.

Now with the trusty tahr, I always need to click on the Variey indicator, click Play Back, then Next, to change to a new wallpaper.

Do you have any idea to fix this ??

Thanks in Advance !!