WP_User_Query not searching

WP_User_Query searches the $wpdb->users table. It will not join on your custom table. How would it know what to JOIN? The possible tables names and structures are practically infinite.

I believe you might be able to use a filter on pre_user_query to insert your own values in the WHERE clause but there is no JOIN value so you will have to alter query_from and query_where. It could be tricky but should be do-able.

Edit:

Given new information, it now sounds like what you need is a meta_query.

$args = array(
    'count_total'    => true,
    'fields'         => 'all_with_meta',
    'meta_query' => array(
        array(
            'key' => 'user_business',
            'value' => 'Clark',
            'compare' => '='
        )
    )
 );
$user_query = new WP_User_Query( $args );

That assumes that your user meta key is user_business and that you want an exact match. See the Codex for other values for compare.