Windows sorts registered overlay icons alphabetically in the registry and only uses the first 15 of them. If you have other applications installed that use icon overlays on files and folders, those applications may take priority to display their icon overlays causing your Syncplicity icon overlays to disappear.
Syncplicity automatically detects the icons are not visible and provides you with a notification that allows you to display the Syncplicity overlay icons once again. Simply select Fix overlay icons when you are presented with the notification screen.