Try an expanded query as your current query is relying on the slug of the tag.
$args = array(
'posts_per_page' => -1, //get all posts
'post_type' => 'post', //assuming your are not using a CPT
'tax_query' => array(
array(
'tag' => 'homepage' //select your tag
),
),
);
$casestudy_query = new WP_Query( $args );