Mac Mail emails display no content when opened
I am using Mac Mail 14.0 with OS X 11.6 and iCloud. Often my mail messages, when opened, display no content, no text, no message, no attachments -- just absolutely empty. Complete restarting of the Mac sometimes seems to help, but not always. My internet connection is of medium quality (Comcast) but I detect no other possible internet issues when Mail acts up.
I'm foxed; any suggestions
MacBook Pro 13″, macOS 11.6