add_rewrite_rule() not working

I have created a rewrite rule, that seems to be registering, but simply won't work. Copied the below from the Wordpress forum, where I've also asked the question:

I have a list of properties on the home page http://example.com. If I want to view a property in detail, I send it to the page http://example.com/cottage-details/. This page has a function on it that searches for a property from the database. In order to know which property it needs, it needs a propref. I want it to take the propref from the URL so it's nice for SEO eg, http://example.com/cottage-details/G368/. As far as I can tell, the add_rewrite_rule() is how I should go about this. I have installed this plugin: https://wordpress.org/plugins/monkeyman-rewrite-analyzer/ and it's showing that my rule should be working, but for some reason, it isn't.