Valid SQL query return empty

There are view issues with the query:

• First what the @janh wrote.
• Second why you removing the am/pm when
  you could easily use the date("Y-m-d H:i:s");
• Third, there is this
  little helper for the database table prefixes $wpdb->prefix
• Users table the column name is display_name
• Also when you make aliases is a good rule to stick with them through the query even if there are not multiple columns with the same name.

For example without counting the bp_xprofile_data table that I mocked:

$reply_id = $_REQUEST["id"];
$postId   = $_REQUEST["post_id"];
$data = date("Y-m-d H:i:s");
$mysqli_query = "SELECT p.ID, COALESCE(w.value,a.display_name) value, p.post_title, p.post_date, p.post_content, p.post_author, a.user_login ";
$mysqli_query .= " FROM {$wpdb->prefix}posts p ";
$mysqli_query .= " INNER JOIN {$wpdb->prefix}users a ON a.ID = p.post_author ";
$mysqli_query .= " LEFT JOIN {$wpdb->prefix}bp_xprofile_data w ON w.user_id = a.ID ";
$mysqli_query .= " WHERE p.post_type="reply" ";
$mysqli_query .= " AND p.post_parent = %d ";
$mysqli_query .= " AND p.ID > %d ";
$mysqli_query .= " AND TIMEDIFF(  %s, p.post_date ) <  '00:00:15' ";
$mysqli_query .= " AND p.post_content IS NOT NULL ";
$mysqli_query .= " ORDER BY p.post_date ASC  ";
$mysqli_query .= " LIMIT 0 , 5";

$reply_query = $wpdb->prepare($mysqli_query, $postId, $reply_id, $data);

$results = $wpdb->get_results( $reply_query );