Why do Ctrl+F2 and Ctrl+F3 sometimes stop working?
I think I have figured this out:

ctrl+F1 apparently turns keyboard access on/off, which disables the other shortcuts. So I have now disabled this shortcut and hopefully the others will no longer get accidentally disabled.

If this happens to you, try ctrl+F1 to re-enable. Also you can disable this in:

 System Preferences Keyboard Keyboard &amp; Text Input