For new strings (like custom fields) to register – you must have the option to register new strings on.  We recommend to keep it only when you’re logged in as admin as this is a setting that will slow down your site if used on limited servers.

wpml string

Also, since 3.0 version new strings register in the list when you access a page with those string on the page from front end. This means you must check all features and amenities to a property – and then you access the property in front end. The strings then register in STRING TRANSLATION.