pop3 from gmail not downloading all messages

pop3 from gmail not downloading all messages

I have successfully set up pop3 downloads of all email messages from a google apps account.

Unfortunately zoho mail did not download all my old messages. After a week, I deleted the old account and started again. After 3 days, again this stopped at 2009. I have downloaded 1637 emails, but there are lots of emails I have yet to receive.
 I did not get more recent emails that I can see in my google apps mail account.

Why did the download stall? And what can I do about it?

The username and password is definitely right, and I can email out.
I checked server settings. Under POP account customisation: 
Download message from mail server for every  minutes (should be between 3 to 60 min)
Number of messages per fetch  (should be between 20 to 200.)