It was due to css conflict, I’ve added the following code at the end of your active thneme’s functions.php to resolve it:
add_action("wpdm_modal_iframe_head", function(){
?>
<style>
.modal-open #wpdm-locks{
display: block !important;
opacity: 1 !important;
}
#wpdm-locks .modal-content{
padding: 40px 10px 10px !important;
}
</style>
<?php
});