There are two considerations to pursue: one is the online gmail account to review which folders are set to be viewable in an email client. The other is to highlight the account on main pane, rightclick and select 'subscribe' to see which folders are allowed for use in Thunderbird. Personally, I avoid AllMail since often causes confusion, is a duplicate of all messages, and ties up space on the drive. But that comment is just an aside.

Thank you for your suggestions. I have IMAP enabled for my entire Gmail account. Until the most recent update, I have never had any difficulty viewing Gmail messages in Thunderbird, in the Gmail folders to which I have assigned them. I use "All Mail" to find messages that are mislabeled or otherwise hiding from me. Oddly, after creating a new folder and copying into it, from "All Mail," some of the emails that weren't displaying properly in the folder to which I had originally assigned them, the problematic emails displayed properly in the new folder. So, I copied everything in the old folder into the new one, deleted the old folder, updated my message filters, and voila, problem solved.

I'm glad all worked out okay. Thanks for update.