Debit orders don't go off.

Since I have joined Mtn many years ago my outstanding balance goes of with debit order. In Jan 16 it didn't go off and I didn't not check so what happend in Feb 16 they deducted a double balance which is unfair to me. March I checked and saw they didn't do a debit order on the 15 like they suppose every Month. I phoned them on the 22nd an said to them please make sure you guys deduct your money I don't want a double deduction the next month. They said it will be detected on the 23rd March 16 well its the 24 March and nothing has been done I am sick and tired of this bad service that I get and this bad network of mtn. At the end of the day they want to deduct twice their money in one month and then I must suffer and all they can say is you are sorry well I am sorry that I have a account with you guys.