Incorrect billing due to unexplained system glitch

I was really confused to see my debit order from MTN go off incorrectly. After checking my statements I realised that they had under charged me on current invoice and on the next invoice they are making up for the shortfall. I contacted customer service to find out what went wrong and was told there was a glitch on the system. I asked what caused the glitch but could not be given an answer at the time. I was offered the option of logging a query after which I would have to do my own follow ups. I find it extremely unsatisfactory that MTN would not assist me further and left all responsibility on my shoulders. The mistake was theirs and I should be given the courtesy of an explanation and MTN should accept responsibility