Checkdisk Stopped Working On XP Pro

I use chkdsk utility in XP Pro perhaps once a week, it has always worked
from the 'run checkdisk the next time Windows Starts' option in Properties
on the Drive letter. Ya know; it can't run a 'fix' until you reboot, but
will run a 'check' without fixing which is not logged.. Until today, it has
always worked without incident, all of a sudden, even with a command line
prompt, i can't get it to run and I am baffled , even after searching the
Web for a reason why this might be so(?) As i said, a basic check will run,
and is not logged so i can't read it of course, but no 'fix file system
errors..'
Please advise. Thank you MSVPs