Note: This faq applies to Multi Monitor Wallpaper, or Perfect Wallpaper. 
The steps are the same for either app.

There are a handful of common issues. Fortunately - I haven't come across something we can't fix yet!

Wallpaper won't set for one or all screens

Whatever you do, it just doesn't set the wallpaper correctly.

You have tried creating a wallpaper manually, and the autochanger.

Either one screen - or all the screens simply don't change.

Wallpaper is lost/broken when you attach or remove monitors

You can set wallpapers ok, but when you attach or remove monitors, they either lose the wallpaper, or it is scaled wrongly.

Autochanger doesn't work

You can set wallpapers manually, but the autochanger doesn't change.

You can right-click on the menu icon, or click the 'change now' button - but nothing happens.

You're using a folder on your computer as a source of wallpapers

Note: This fix applies to Multi Monitor Wallpaper, or Perfect Wallpaper. 
The steps are the same for either app.

This fix typically works if:

What to do

This will reset all your spaces/desktops (though you won't immediately see the change in your currently visible workspace)

Background for the Technically Minded

Setting the wallpaper is actually a very simple command in Mac OS. Essentially - I say 'set this image for screen N'

Once that is done - Multi Monitor Wallpaper or Perfect Wallpaper has no part in 'keeping the wallpaper active' - the operating system is completely in charge.

This means that when the wallpaper is 'lost' - the operating system failed to properly save and restore the configuration.

In some cases, the system gets 'stuck' and stops allowing apps to set new wallpapers.

I have pinned down at least one way to reliably re-create this bug on my mac. What seems to happen is that the database where the system stores what image is applied for each screen/space gets corrupted.