If you have found the email, that is not filed into the smart folder you created, select the email or click on it. Then go to the top of reMARK, under “TOOLS” and click on the button “Match email to filter”, which is in the top left corner. 

Now select the folder, the email was supposed to be in, and click match to filter at the bottom right corner. 

It will now show you why the email isn’t getting picked up. If however everything is matching, it could have removed from the folder by a user. Try to enable Team Overview with the “View” tab, to show you the actions made on this specific email.