get_template_part returns NULL

As the codex page of get_template_part() says:

get_template_part doesn’t return a value and doesn’t warn if it fails to find a matching template file.

Additionally:

If you want to hear about failures, use:
<?php assert( "locate_template( array('$name-$slug.php', '$name.php'), true, false )" ); ?>

So no wonder you are get nothing back.

One thing I’m seeing is that your are adding the file extension to the $name parameter. Which essentially does mean you are looking for files named: slug-name.php.php, I doubt that is what you want. The filenames are constructed like this {$slug}-{$name}.php, which means get_template_part() adds the file extension automatically, as you can discover by looking into the source, aside from that it is explicitly stated at the codex page.