i have new gmail messages but thunderbird tells ne there is no new mail

I have new emails in my gmail account that I have verified as existing and dated 5/13/16 (last message of the day), 5/14/18 and 5/15/16. But when I try to download these through thunderbird, I and told that there are no new messages. i.e. I cannot access my gmail messages using thunderbird. Earlier messages on 5/13/16 were downloaded in the normal manner. No changes have been made to thunderbird or gmail setings