Sky Mask 'Sky 1 could not be loaded'

When the XMP file for one picture is copied from a MAC to a Windows PC (and reassociated with its corresponding CR2 file) and opened in Bridge the Sky mask layer is present but it has no affect on the image: a small yellow warning triangle says 'Sky 1 could not be loaded'. Other brush masks have loaded on the same image without problem. Can anyone suggest a reason for this please.