You've marked a file or folder for offline use, but now you can't see it in Windows Explorer. Why?
In Windows XP, this usually occurred because the file is still in use—an open file handle is therefore preventing the file from synchronizing properly. Note that this can still happen in Windows Vista too.
In Windows Vista there's another possible reason however, and that has to do with background sync. In Vista, offline files sync in the background when user activity is low. This change results in faster logoff times because you no longer have to wait for offline files to sync when logging off. But it also means that at any given time, a sync operation may be happening in the background, and as a result some files may be temporarily unavailable. If a user has a large amount of offline files, users may need be educated to wait for sync to complete before their files all become available. You can also enable the "Synchronize all offline files when logging on" Group Policy setting and use this to force a full sync when the user logs on, and then tell users who have tons of offline files to log on and go have a coffee while their CSC cache synchronizes.
If you have feedback concerning this tip, I'd love to hear from you.