Problem seems to come rather from Qpopper. Which authentication method do you use ?
If your message is in /var/spool/mail/username, and you can't get it, then look at your Qpopper configuration, to check which directory it checks in.
I think it's the key, if your messages reach the right dir, then Procmail has nothing to do with it. Otherwise, let's blame it while looking at your conf file and log messages...
J
You can lean only on what resists you...