|You should run chkdsk /r |
This will check the filesystem for integrity and fix (minor) problems.
Unless the filesystem is 'clean', defrag will not run. This is a safety check built into the defrag API.
After the chkdsk is done, if you're still having trouble, then also try with another defragger ....download a demo version of a reputed third party auto defrag utility and see if runs properly.