Depends on whether you created a pop or imap mail account and whether the downloads have reached some limit imposed by the server or whether a log jam has occurred. If POP then Thunderbird is only allowed access to download whatever is in the server 'Inbox'. If IMAP then perhaps you need to subscribe to see other folders. If this is pop and a log jam has occurred. Logon to webmail account via a browser and empty the 'Spam' folder. Then in Thunderbird click on 'GetMessages'. If still no joy then in webmail account, read and delete the oldest email that did not download. Then in Thunderbird click on 'GetMessages'. If you have reached a downloading limit then wait a few hours or possibly the next day, do not delte the account as it is unecessary.