Just had a quick look in to this and it appears that its linked to the way the forum see's people as being inactive.

We have the forum set to inactivity based, which means after 15 minutes (current setting) of inactivity the forum clears you from the system, (Logs you out) and resets your settings, including new posts. So new posts will only show from when you were logged out.

I have upped this to 30minutes, so see if this improves things for you

There are other settings that can be used but they are all server intensive! which until we can sort out the other issues, we don't really want to use.

When we have the forum running better, I will look into changing from the Inactivity based setting to one of the others. (i will make a note in the admin panel)