Non-SPAM emails going to Trash/Junk, getting stuck there

In Mac Mail, I discovered that some of my legitimate, non-SPAM emails are automatically being filtered into the Mail Trash & Junk folders. I can't find any "Rules" or settings that would cause this, & also couldn't find any causes in my gmail settings. I had made sure that each of the senders' emails was already entered into my email address book, so that they're not wrongly marked as SPAM by the email system, but it hasn't solved anything. Usually I can go through Junk & manually move the non-Junk into the Inbox, but sometimes they get stuck in Junk. Same with the Trash folder in Mail. I even tried logging directly into gmail from a web browser, removing the "Trash" label & manually moving them to the Inbox. Both via Mac Mail & via the gmail-login, the moved messages would momentarily disappear from Trash, but within minutes would be back in Trash. How do I fix this?