The {index} tags are not being converted

The Quick Index plugin is not published

Check if the Quick Index plugin is published. Go to Extensions >> Plugin manager and search for the system plugin System - Regular Labs - Quick Index. Then publish it.

There is markup on the {index} tags

If you have copy/pasted code directly into your editor, you might also have copied along hidden HTML tags. So this will also cause the {index} tags to break:

{<span>index</span> ...}

Remove the extra styling or other html tags around and in the {index} tags that might cause problems.