I think I fixed this on my own.  I had a rule set up in Outlook that cc'd
every email I sent to my email account, including meeting requests.  It meant
every time I sent a meeting request it added me as a participant for the
meeting as well as the meeting organizer.  Then when I accepted the meeting
request to keep it on my calendar I became a participant only and could no
longer make updates to the item.  I solved this by modifying the rule to not
copy meeting requests back to myself.  All seems to be well now.