LJSNYC wrote:

Why is it that some mail in my inbox and in folders is not immediately
available when clicking on the message? Instead, I have to wait for
it to download.

Because that's you configured Outlook.

Is there a setting I could change so that all mail remains fully
downloaded at all times?

Modify the settings in your Send/Receive group to download your messages
when Outlook performs a send/receive.
--
Brian Tillman [MVP-Outlook]