Check the message logging of each interactive joblog and verify they’re the same.
If the user did a RNMOBJ and it failed, they would have received the error on their screen.
What exactly does it display immediately after the failed RNMOBJ in the joblog?
<i>This rename was not successful. Why has a message not been sent?</i>
Assuming the first sentence is true, then the message <i>was</i> sent. The fact that it’s not in the joblog isn’t evidence that it wasn’t sent.
It might not be shown because it’s below the logging level for the job or some process might have removed it, e.g., with RCVMSG, from the program message queue that it was originally sent to (or from the *EXT message queue of the job which is what usually gets referred to as the “job log” before the spooled “job log” is created).
(Note that “Message queue SIMON” is unrelated. SIMON is most likely a user message queue rather than a program message queue. SIMON <i>could</i> be a device message queue or other *MSGQ object, but it doesn’t seem likely. In any case, it definitely isn’t a program message queue and has no relevance to a joblog.)
In order to answer your question fully, we’d first need to know the LOG() level of jobs that show the message and jobs that don’t. If that provided no answer, then we’d need to know what programming was executed after RNMOBJ and before viewing the joblog.