The Task Scheduler relies on your DNN internal scheduler to execute tasks. By default, the DNN scheduler employs a "Request method" strategy, which relies on someone to visit your Web site to trigger an action. If your Web site doesn't receive a lot of traffic, you may see a delay with the scheduler until someone visits your site. You may also subscribe to a keep-alive service to continuously ensure activity on your site.
Alternatively, you can set the Scheduler mode to "Timer method" under the Settings > Scheduler. The timer method allows the scheduler to run separately on its own living thread outside the reliance of the user request. Please note if your site is idle for a very long period of time, the IIS system will attempt to conserve resources by terminating the Web site process including the timer thread. In this case, you may want to extend the idle timeout (default 20 minutes) on your IIS system.