It sounds very strange. The only reasons I can think of are:
(1) Your computer was not running at the scheduled time; e.g., it was turned off, entered standby or hibernate mode, or was disconnected from the Internet.
(2) The task might have run; however, there were no file changes.
Please try to create another small backup task to test it further. If you have more problems, please email more detailed info, incl. screenshots to DriveHQ customer support.