File system shouldn't really matter. Sometimes Defrag can't move these files for many cases. Most common resolution is to do a disk check (scan). You can do that by right clicking on the drive and properties, just like you do defrag.
Do a scan, there might be some corrupted files on the drive which should get fixed. Once that's done, try defragging again.
You may have to defrag your drive 2 or 3 times before it'll be able to move all files, or most in some cases.
Hope this helps.
TIP: You could disable the screensaver to help the procedure.