Users with custom roles can’t read each other’s comments

I had the same issue, main users have the contributor role and a simple sidebar comment sidebar widget was not working when logged-in! Strange. I solved it by some sql:

function getLatestComments ( $postid ) {
    global $wpdb;
    $sql = "SELECT * from $wpdb->comments WHERE
             comment_post_ID = $postid AND 
             comment_approved = 1 
             ORDER BY comment_ID DESC";
    return $wpdb->get_results($sql);
}

I used this instead of get_comments. You may modify the sql and customize as required.