Remember: Publish Search Results page after modifying it or your Search Refiners might not show up for other users!

When you edit the Search Results Page of your Search Center:

To modify the Refinement Panel Web Part to add Custom Refiners:

And you save the page after you are done:

Don't forget to publish the page!

(Or if you don't have enabled Publishing: click Check In right next to Save & Close)

Otherwise other users might not see the search refiner. You will probably notice this when testing with another user's account. Your new search refiners which so far worked flawlessly suddenly do not appear at all.

So if your search refiner shows up under the account you created it but is not visible anymore when logging in with another user's account you might have forgotten to publish the page.