Account not debited

I would like to know why my debit order did not go off at the end of April ? I have already paid manually , End of March the debit order went through but this month it did not go through . This is getting frustrating because you guys have been doing the same thing last November and it is affecting my credit score . I have had a back and forth with yet there seems to be no change , I received an SMS reminding me that the debit order will go off on the 30th of April , but nothing went off . Then before I knew it I was told my account was in arrears . I paid manually like I have always been doing when you do not run the debit order, what is it that I have to get you guys to do the right thing ?