|Perhaps some services that are essential for defrag have been disabled? Under the services list check on and set the following to autostart: Task Scheduler, Disk Defragmenter, Remote Procedure Call (RPC), DCOM Server Process Launcher, RPC Endpoint Mapper.|
Also, you may want to run chkdsk /r to scan for, and recover from disk errors. If the filesystem is corrupt defrag will not run.
As for the free space, 90% is plenty. But just FYI, the 15% requirement is only for the default Windows defragger. Newer third party auto defrag utilities are designed to work even in 2-3 % free space.