That sounds like a caching issue... Try holding down your shift key while reloading the page (or delete your browser's cache manually) to see the latest version.

If you're behind a network proxy (at work or at school), it could be holding pages, in order to serve them faster to you and others on the network.

Your posts go right into the system, which is why the post count increments, so they're up there immediately. The problem is that thread pages are basically HTML, which may end up being cached on either your computer or the network you may be behind.