GMail and POP3 RETR problem - switch to IMAP?

This is intended behaviour of Gmail. According to this question, "[a]ll messages may be downloaded to another computer once; after downloading mail, it will not download again."

There's also a 'recent mode', in which the last 30 days of mail are fetched, regardless of whether it's been sent to another POP client already.

That said, don't try to fetch all your mail by different computer in a short period of time, as Gmail may block your account for 24 hours.

I strongly suggest using IMAP.