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

Multi Monitor wallpaper provides multiple ways for you to adjust your screen layout. From easy dragging to very precise keyboard controls. You can customise the layout, as well as details about your screens.

Layout

1) Easy Layout Buttons

Use the layout buttons to get started.

The System Layout simply copies your default system layout

System Layout + Spread uses the system layout, but expands things a little so that there is a gap between your screens.

You can use the alignment buttons (up, down and -) to align the tops, bottoms or centres of your screens.

2) Just drag the screens

You can click on the grid background for each screen and simply drag it around. This is the easiest and quickest approach if you need to adjust things manually. You can use the horizontal and diagonal lines to line your screens up pretty accurately.

You can click on the grid selector at the bottom of the window to change your grid type

2) Drag in the window

Drag the screens in the main window as an alternative method to adjust their position

You can hold down the shift key to lock the movement in the horizontal/vertical directions

3) Fine Control - Keyboard

You can use the keyboard for pixel-by-pixel adjustments.

Click on a screen in the main window, then use the arrow keys to move it.

You can hold down the keys but be patient, you're literally moving a pixel at a time!

Screen Configuration

Normally, the system gets the size of your screens about right and Multi Monitor Wallpaper will use those sizes.

Sometimes though - the details are a bit off, and you might want to adjust them manually.

Simply click on the dropdown to pick the screen you want to adjust (you can see the numbers in the main window). Click 'Adjust' and you can manually enter the width in mm of your screen.

Get Creative

You can get creative with your layouts!

For example, one user has a row of six screens in a grid, and a separate screen on the other side of the room.

Your layouts don't have to match the real world. I advised him to set up his 6 'grid' monitors in a normal grid pattern. Then to adjust the size of the 'extra' monitor so that the system thought it was really large and put it 'on top' of the other monitors

Grid in Grey, 'Extra' in red

This way, the grid monitors get a wallpaper spread across them in the normal way, but the 'extra' monitor gets almost the whole image as the background.

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.

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

This fix typically works if:

Before you try this fix - you might want to try Reseting the Wallpaper Database first.

What to do

This fix makes sure that MMW or Perfect Wallpaper has access to the folder where your images are stored.
It does this by temporarily allowing the app to access any folder on your computer.

(step 9 is optional, but it is a good idea as the app doesn't need this access generally)