Perhaps this post can be helpful.
If not, then here are a couple of other things you can attempt:
Override in parent-theme
You could also (temporarily) try to override the template in the parent-theme, to see if that works.
Double-check uploaded files
I would probably also double-check the files on the remote-host, that the isn’t some old file(s) messing with the setup. I don’t know about you, but I sometimes just delete files locally and forget to check if they’ve been uploaded.
Delete template-folder from the remote host and reupload
… You could also try and delete the entire template-folder on the remote host, and then upload a single file (in case some file is cached/corrupt/whatever).
Incognito
Perhaps try in an incognito-tab (in case of cache).
Use die()
In case you try and print something and can’t find it, then perhaps try and insert below-written code-snippet in your template, since that makes it obvious if the template is being executed or not.
echo '<p>Do you see me?</p>';
die();