An unexpectedly short message list can be a view problem. Thunderbird's Quick Filter can restrict a folder to unread, starred, tagged or other matching messages. Its Pin control can retain those restrictions when you change folders, making several folders appear incomplete in the same way.

Inspect the list you are actually viewing

Select the account and physical folder where you expect the message. Open the Quick Filter bar from the message-list header. Record any typed search and active filter buttons before clearing them, especially if someone relies on the current view for daily work.

Remove the text restriction and turn off the selected filter criteria. A search can be limited to fields such as sender, recipients, subject or body; a remembered word in the body will not match a subject-only search. Check the Pin state before moving to another folder.

For a controlled comparison, find one harmless message that is already read and has no star. It should disappear when an unread-only restriction is applied and return when that restriction is cleared. This demonstrates a display rule without changing the message or its storage location.

Restore the useful filter deliberately

If the missing item returns in the unrestricted list, record which criterion excluded it. An attachment-only view, for example, may omit a message containing a cloud link rather than an attached file. That observation does not imply a delivery failure.

Reapply only the restrictions needed for the task. When filtering tags, review whether every chosen tag must match or any chosen tag is sufficient. Use a plainly named workflow note if several staff members share responsibility for the mailbox, so an intentionally narrow view is not mistaken for the complete account.

If the message remains absent after clearing Quick Filter, continue with the original folder and provider investigation. Avoid removing the account or rebuilding its data solely because a list was short. The useful result here is a demonstrated link between one view setting and one known message's visibility.

Source: Thunderbird documentation.