Hi,
If the idoc is set up in such a way that in case of error feedback occurs using workflow.
Then probably the user is not a recipient of the workitem.
Check the workflow log to see if these workitems even exist, you can also try transaction swi2_adm1 to find workitems without agents.
Solution is then to either change the agent assignment of the task, or it could be authorization issues with the users themselves.
Kind regards, Rob Dielemans