When a workflow fails during a modeling session, you can click Approve in Model Changes, or Reload the changes and approve in the Modeling Session view. If you approve, PM Compass automatically repeats the last step action and continues.
If you access the same workflow in Workflows » All Workflows » <select workflow> » Form View, you can use the Advanced tab to see why the workflow failed. After you correct the problem, you can click Workflow Actions » Continue Approve to complete the workflow.