This could be related to several things. First, the file might be locked. Usually, a defragger tries several times to move the file and finally skips it. Another reason could be that there is not enough continuous free space left and the defragger tries to free some space by moving other files out of the way. Those are the most common reasons. To give you a more founded answer, we need to have a closer look at the specific configuration of your system.
This was first published in February 2002