WP Smart Sort plugin won’t save preferences

I’ve been using WP Smart Sort on a couple of WordPress based websites. On one of them, when I try to add a new sort field via the WP Smart Sort management page in the dashboard, the site won’t save the new field. I.e. after I click ‘add’ nothing happens. Usually the ‘add’ button would change to ‘remove’, showing that the field has been added.

The problem doesn’t occur on the other site that uses the plugin even thought the hosting package and settings are the same.

I’d be very grateful for any help with this.