Restart of WI not possible (WI 726)

[Robin Sahasranam]

One of our workflows (release purchase requisition) got a role resolution error.
After fixing the cause of the error, I attempted to restart the workflow from
transactiion SWI8. However I get the message WI 726 Restart of workitem & not
possible.
 
Any suggestions on what I need to do to restart the workflow? Under what error
conditions is a workflow restratable?
 
Thanks in advance for your help.
 
Robin Sahasranam
World Bank