The below are the few reasons why the scheduled process chains are not started executing,
a. The Factory Calendar, only the working days defined in the Factory calendar will considered for process chains (Already Scheduled process chains) execution.
b. The Authorization, when the process chain is scheduled by a User XXXX, and if the authorization is removed for scheduling or executing process chains for the User XXXX, the next day process chain will fail to execute as the authorization is removed for the User XXXX. So it is recommended to schedule a process chain always with the User “ALEREMOTE” as this user will have SAP ALL access and there are no chances of removal of authorization.