I've just installed Thunderbird, hoping to create a local backup for several years' worth of email on yahoo's server. It dutifully downloaded the last two years' worth... and then stopped. It continues to pick up any new messages that arrive, but nothing I do will get it to download any more of the past messages. What can I do to finish the process?

Very appreciative for any advice! I've been using email for decades, but am entirely new to storing it locally.