A parent folder selected for Thunderbird IMAP synchronization does not establish that its children are selected too. This is easy to miss after creating a new project subfolder: the parent works as expected, while messages in the child require a network fetch. Diagnose the folder tree before changing the account's login or rebuilding its data.

Trace the exact hierarchy

Write down the complete route, such as the account, Projects, then the relevant project name. Identically named subfolders can exist under different accounts, so include the account at the beginning. Compare one affected child with a sibling that behaves normally.

Open that account's Synchronization and Storage settings and inspect the Advanced folder list. Expand the parent and read the child's own selection state. Mozilla's documentation explicitly treats these as individual selections. A tick beside Projects should not be transcribed in a handover as every project folder covered.

Check whether the account's general message-retention choice or an age/size limit also narrows what is downloaded. These are separate questions: which folder is included, and which messages within it qualify. Changing a date limit cannot compensate for a child folder that was never selected.

Correct the omission you found

Select the required child deliberately and allow its synchronization to finish. If several nested levels exist, inspect each relevant entry. Avoid selecting unrelated personal or historical folders simply to make the tree look uniform.

Compare the affected message body before and after the change during an agreed offline reading check. Leave sending controls alone. If the body still needs the server, retain the precise folder and message characteristics for support instead of assuming the parent selection has now proven everything below it.

Update the folder inventory to include the newly selected child. When another subfolder is added later, revisit its own setting. The useful outcome is a documented correction to one missing branch of the synchronization tree, with a message from that branch actually readable; it is not a claim that every message in the entire account has been downloaded.

Source: Thunderbird documentation.