Are you using outlook on your computer? If you are, in Outlook you need to Tools>Account settings>double click on the e-mail account>More Settings (bottom right)>Advanced> make sure leave message on server is checked! 10 days is a good setting!
What is happening is that Outlook is removing the mail before the BB server can pick them up!
I was having the same problem! After 5 calls to tech I got a brilliant young lady in Tenn. who knew immediately what the problem was.