If it's six months old - they yes - it might be a reason for concern as users might be set to the old domain. Can you check and see if you are blocking with robots.txt the old domain some how ? Since if that's the case the bot can't reach the old pages and see the redirection and if those pages are already in the index they will stay that way.
Alternatively check the logs and see if google bot did hit those pages in the last 6 mo - although I doubt it didn't - it's safe to check.
Cheers !