Find Usages not finding anything except open files

Hi, I am a new user to PHPStorm. Having just got set up I was excited to start using it.

However, on my first file i wanted to change a section of html which i know is repeated in several documents. However, using 'Find Usages' for the CSS class on this particular element is finding nothing except the open document!

I have searched for answers, and tried 'Invalidate Caches/Restart' twice to no avail.

In fact, even if i select a class which is repeated in the same document many times, it's only finding the usage where I start from.

This is very frustrating!

FYI the settings for 'Find Usages' is set to Project Files. I've also tried All Places, as well as a custom scope.

Any help would be very much appreciated! :(