What code enables RSS feeds in WordPress?

I have dozens of active WordPress websites, most of which automatically enable active and valid RSS feeds

Some of these sites do not have usable RSS feeds ?!?!

Search indicates first probable cause are plugins. In each case, I have disabled ALL plugins and RSS feed is still broken

Search indicates that second probable cause is the theme. I have switched themes, always to the default twenty somethings; this morning to twenty fourteen – again, NO usable RSS feed

On shared hosting sites, most shared hosts have active and valid RSS feeds; whereas one shared host does NOT have a usable RSS feed – ON THE SAME HOST

So, WHAT is responsible for a usable RSS feed on a WordPress site?

Please, help me understand how to fix this annoying anomaly. Thank you.