Periodically, couple of times a week, I will open an email with fill header information but a blank body. There is a message that it is offline and will download during next online connection. It will never download, I have to go to Gmail to read it. Emails on either side of the one that is blank appear properly.
Could be because the message cache is corrupted. For GMail or GSuite accounts, right-click on the All Mail folder and choose Properties > Repair. For any other IMAP accounts, right-click on the affected folder.