It is possible that one of the e-mails waiting to be downloaded is corrupted or malformed in some way. That e-mail is acting like a "clog" in the pipe, so to speak, because when Thunderbird encounters it, it can't deal with it. So quit Thunderbird, log into your webmail and check the first/oldest message. Open it, then either delete it or move it to another folder. Then run Thunderbird and check your mail. If it still fails to download the e-mail, go back to the webmail and do the same with the next oldest message.