This issue can be caused by a build up of idle tasks which also stop the maintenance from running each night, which in itself, compounds the problem. Idle tasks are tasks which have completed but which have not cleared down the "iwtskrun.exe" process on the server.
This can be prevented by adding some registry keys on the server, which ensure the process does clear down.
Please raise a new case online and reference the title of this article and we can then get these added for you, or provide details to an experienced member of your IT team to add themselves.