Are you using remote desktop?
If yes google for "remote desktop copy and paste not working".
I also thought the problem was from pl/sql developer, but if you disable in Remote Dekstop options -> local resources -> untick clipboard, everything works properly (but you will not have the ability to use copy/paste between remote and host computers).
I also found this:
http://blog.sqlauthority.com/2008/0...-remote-desktop-copy-paste-stop-working/but still didn't try it out.
Cheers,
Manel