message will not display when selected. This just started two days ago and occurs for new emails and messages in my local folders. This is a POP3 setup. All I get is little dots moving above the subject line.
Usually POP accounts download the whole message content when the message arrives.
What you describe is more common with an IMAP or Exchange account. What that does by default is to download only the message header, then when you click on the message it retrieves the content, with the in-progress dots moving across the screen.
Or, with a POP account, it may be caused by linked content. In that case the POP message content cannot be download when the message arrives as it may be live or changing content and not embedded in the message. When you open the message, it begins to download and you will see the dots.
If this is happening with only some messages, I would not worry too much.