The most common cause of tasks only sending when running manually from within TaskCentre and not as scheduled to automatically run, is that the Access TaskCentre Schedule Agent service has stopped running.
The service can stop running after a server crash or reboot, amongst many other reasons, and so is a good first thing to check when tasks do not send on their automatic schedule.
To restart the service:
Log on to the server where the Orbis Task Centre is installed
Open up Control Panel and then Administrative Tools (steps to find these will differ depending on operating system)
From within Administrative Tools, double-click on Services
In the window that opens, find the service called "Access TaskCentre Schedule Agent"
Right click on this and select "Start", or if that is greyed out, "Restart"