Debit order dates

So what is the point of submitting a date for the debit order if the order only goes off after that and then bounces. This happened to me and my husband on various occasions. And after the third month of the debit order going off on the wrong date they made the annuity paid up. They reinstated it after we sent in new docs wit proof of payment for the months missed and this month again they debited the accounts on later dates than stated on the application forms. Please explain why???