robots.txt not working

WordPress has a dynamic robots.txt file which I understand should be overridden if you upload a physical robots.txt file to the server (in the root directory).

I've done that, but when I go to the domain/robots.txt it's still returning the dynamic default WordPress one. Google Webmaster Tools and other online robots.txt checkers are also seeing this.

Can you help me figure out why the file isn't showing? This is a live site, so I'm reticent to do a theme and plugin compatibility test.