Check that View/Threads is All, and that the Unread button on the Quick Filter Bar is not selected.

View/Threads is All and Quick Filter Bar Unread button is not selected. I also use gmail to manage this same email address. IS there anything related?

Is the account POP or IMAP, and is it accessed by another computer or device, e.g. phone?

Hello sfhowes, I have chosen IMAP account configuraton and yes I also use another device to access the email.

re :yes I also use another device to access the email.

Is that other device also using IMAP because it sounds like it is using POP. If other device whether phone or computer is using POP and you have not set it up to 'leave message on server', then when you use it, emails are popped off the server. This means Thunderbird imap account cannot show something that does not exist.

Log on to webmail account and check what is on server. If it is not on server - then it will not show in imap account. Then check the set up on other devices.