first of all try:
1. stop guest
2. Settings in VirtualBox GUI
3. General | Advanced tab
4. Look for settings "Shared Clipboard"
Is it set to Bidirectional? If yes then it is ok.
Sometimes copy/paste between host and guest and vice versa is also not working on my PC, but I can't reproduce the problem whenever I would like. I just stop guest and restart host.
If you can reproduce the problem then report bug to Public BugTracker:
P.S. BugTracker is using its own login, so you have to register first (link at the top of window).