If you receive many mails from a form in a web page, and all of them have the same subject and have a small time difference, the Gmail puts all of them in a single thread. If you need to track the answers of each user individually, it becomes a mess.

(One possible solution is editing the formail.php to hnge the emails subjects and make them different, for example add the submitter name, the submission hour. Last time, I just added a random number, it's slightly confusing but it solves the problem.)