You wouldn't by any chance be using PuTTY to access the session? If so, you may need to play around with the terminal settings including the scroll type so that it displays correctly. I don't recall the specifics but a similar thing happened to me.
Actually, no I'm using gnome-terminal on Ubuntu 10.10. I wonder if it's due to the fact that I'm ssh-ing to other machines within each screen window? Sometimes I will do this if I have a dozen servers to work on at the same time, I have a little script that spawns a new ssh session to each box in separate windows. Here's a little tidbit that I just learned; you can send the same command to all windows at the same time:
[CTRL-A] :at # stuff "pwd; hostname; uptime^M"
That will send the pwd, hostname, and uptime commands to all windows. YMMV.