1. Right-click the account in the Folder Pane, Subscribe, Refresh. Does the new 'label' appear in the list?

2. Right-click the folder, Properties. Is 'When getting new messages....' checked?

I refreshed the "Subscribe" for the account. No change.

1. As far as the new folder (Gmail label: _MOM...), it appears in the folder list in the Subscribe dialog, and it is checked. Interesting thing to note is that the folders (labels) are divided into two alphabetical lists in the folder list. See attached image that shows where one list finishes, followed by the defaults Gmail folders, followed by the second list. It's not even like the list break is the difference between what Gmail "shows" or "hides" - you will see that the folder missing from Tbird, _MOM... (which is a "Show" file in Gmail settings) is in the same listing as _VA_TENN.... (this one is a "Hide" in Gmail) - but this shouldn't matter because none of the folders is "hidden" in Tbird. Also note that the missing folder, _MOM... does not appear in the Folder list Pane on the main screen. See second attached image.

2. I checked the box for "When getting new messages" in one of the existing Tbird folders but it didn't get the messages out of the corresponding Gmail label. Also clicked on "Repair Folder" to see if that did any good but no. I checked a few folders and seen that none have the "When getting new messages" block checked, but recently picked up new adds to the corresponding Gmail label.

What do you advise?

Try changing your gmail label to something without a leading underscore. Just a guess, but my guess is that causes the label to be ignored, or not even sent by Google. You might want to have a look in your gmail settings to see the labels are set to be shared in IMAP. https://mail.google.com/mail/u/0/#settings/labels

Note you can get rid of the [gmail] step by entering [Gmail} in the IMAP server directory in Account settings > server settings > advanced

Thank you.

The Tbird is already accepting labels having leading underscores, but I tried changing the label name in gmail as a experiment. Then I refreshed the folder list in the Subscribe dialog in Tbird. Although the folder list in the Subscribe dialog is refreshed and the revised label name appears there, the new folder still is not in the folder pane of the main Tbird window.

As far as the Gmaill settings, IMAP is checked for the new folder name. IMAP is checked by default when a folder is created.

open the profile folder (help menu > troublshooting information > show profile button) close Thunderbird Delete the file foldertree.json from the profile folder.

Restart Thunderbird. perhaps the file is damaged.