I did find another question with a similar title to my question but there was no solution.
Basically, a flash message I'm setting (and persist is true) is not showing after I re-direct a user to a different page, but the problem only exists with my production environment.
If I use the frontend_dev.php controller, the flash message shows correctly.
I've tried completely clearing my cache (cd cache; rm -rf *) but this doesn't make any difference.
The question with a similar issue is here for reference: Can't use flash method in prod environment
Any ideas?
I've inspected the elements on the page to make sure the flash message definitely isn't there (and isn't being hidden by javascript or anything like that).
Cheers