In WordPress, you can use the get_terms function
to retrieve items from a taxonomy. However, if for some reason this does not work, try the following solution
Set hide_empty => false
as an argument. This will allow you to retrieve terms that do not have a post associated with them. The specific code is as follows
$args = array(
'hide_empty' => false,
);
$terms = get_terms( $args );
This method should give you the expected results.