Tip: if you’re wondering why your WordPress template file (for example index.php) is not being executed, perhaps a different file (archive.php) is overriding it. Now this may sound obvious, but not when you start using get_template_part to organize your theme files, and not when you’re making a child theme.
Let’s take a quick look at the following snippet:
get_template_part( 'content', get_post_format() );
That will look for files like content-link.php, content-gallery.php and so on, and will always fall back to content.php if none of the others are found. Now, imagine you’re working on a child theme. You create content.php and content-gallery.php. You expect gallery posts to pick up content-gallery.php, which is correct.
You also expect a link post to pick up your content.php file because there is no content-link.php in your child theme, right? This is also correct, unless your parent theme has a content-link.php file, which will be of higher priority to the template loader, despite the child-parent relationship between the two themes.
It does make sense, otherwise a simple index.php file in your child theme would override all of the parent theme’s templates, because index.php is a fallback for everything. That would render child themes useless.
So when working with get_template_part (and theme templates in general, thanks Chip!) especially child themes, don’t forget to check (and preferably study) the parent theme’s template files. Also keep the template hierarchy fresh in your head every time you create a new theme template file. It tends to evolve with every release.