Django messages faulty, but only on one particular

Question

I'm experiencing an issue with Django's messaging framework, but only on one specific network. Two users have reported the issue on their office network, but if they access the site via a different network (e.g. their home network), everything runs smoothly.

The issue is simply that messages don't get 'cleared' after being viewed. So once a message has been created, it will persist across requests until the user has lots of messages piling up on the screen.

I just wondered if anyone had any other ideas about what might cause something like this?