If schedulers are left alone for a period of time, eventually the schedulers stop processing. They can completely pause due to lack of manual intervention. Once someone has clicked into any of the scheduler windows, the scheduler starts processing again, which causes a backlog needing to be processed.
There is an ini setting that support can add for you which will set the scheduler to execute.
This is only available from Version 5.4 and if you require it, please contact us and reference the title of this article and please include details of the version you are running and when the scheduler stopping processing.