When you create a new activity in a change request, it is best if the relationships to and from that new activity are saved in the workflow with the new activity. For example, if you are modeling several change requests at once, when you mark a workflow active and create a new activity, create all relationships to that activity while that workflow is active. This will help to ensure that the activity exists during the complete step. It is possible to create a relationship in a different workflow, but you need to be sure that the workflows with the new activities are completed first.
If you have several workflows where you are creating new activities with relationships to the new activities in other workflows, you should make sure the workflows complete. If the workflow is not completing, refer to the Schedule Change log to see why.