If the account is POP, only the Inbox is downloaded to a mail client. IMAP allows access to all the folders. To change from POP to IMAP, the account has to be re-added as IMAP:

http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

https://support.netregistry.com.au/articles/nr/Email-software-settings/

https://support.netregistry.com.au/articles/nr/Thunderbird-email-setup-tutorial

Thanks for your quick response but this is a 20 year old account and Netregistry have left it on old technology and it appears that can only be accessed by pop and not imap, and not converted to their current products which is what your links go to . And i can't convert as i can back up the email nor will it let me get access via imap at this stage - still hassling netregistry re this as i can get my wifes email to work on thunderbird either Total cock up by netregistry and i have 20 years of emails on this account which i a trying to save

As a last resort, if the account can't be converted to IMAP, move the mail in other folders to the Inbox, through webmail, then download via POP in TB.

Great idea but it is not working for all as it only seems to pick up unread messages on the server is there a setting in thundebird i need to change to get these?

Hi trying a different way and clearing the inbox all the time this may be the answer

Not sure if this would work, but moving the mail into the Inbox, then deleting popstate.dat from the profile folder might force a download of all mail, including that previously downloaded. See if you can mark the mail in folders as unread.

http://kb.mozillazine.org/Popstate.dat

I have got most of the mail across now - still have some to do but - saved me a whole heap of issues there so thanks for that