Dragging emails to Tinderbox

This seems to work for some, but not emails. Specifically, some emails dragged in create a note with the appropriate $Title, but with an empty $Text. What am I missing?
Mail 9.3
TB 7.1.0

What kind of emails fail? HTML messages? Images? Mail with attachments? Plain text?

Using Mail.app; I assume they are plaintext; how do I confirm that?
I note also that messages from MailMate always create a $Text that is simply the URL of the message.

Anything that has styled text (bold, italicized, colors) and/or images is probably not plain text.

It takes a while for Tinderbox to fetch the body content of emails, at times.

But if you can reproduce the import failure with specific messages, you might want to use Message > Forward as Attachment and send the message(s) to support at Eastgate to check out.