I have Defraggler scheduled to run on a server a every Saturday and according to the scheduled task log the task is running as expected, however when opening up Defraggler and analyzing the disk it doesn't look like any defragging has actually been performed.
Well you can always schedule a df run yourself through the WTS interface; this is what I usually do as part of a batch file which stops database services before defragging. However I have also encountered the same problem that df does a minimal job in the scheduled run; if I run it again afterwards in an interactive session it runs for a lot longer and does a lot more. I tried scheduling two runs consecutively; the second exits within seconds - but in an interactive session analysis reveals e.g. 27% fragmentation and df runs for for 30 minutes +. The scheduled job runs under a different account - could there be anything there to check?