Users can't see their own posts?

I searched for someone with a similar problem, but couldn't find any, so I posted a new.

When my site's users (only registered users can post) post in shoutbox some of them complain that they can't see their post. Some say it shows after 6 hours for example, while others say it never shows. I did a test with one of them, and it showed up on my screen even though it doesn't show on the users.

I suspects my users internet settings perhaps? But honestly I have no idea what does this. Hope someone can help :)

/Jakob (www.dansktennis.dk)