Atm paid me less

I went to withdraw R8000 rand from the atm and the first atm could only provide R5000. So I went to the second one and withdrew R3000. I came back home and there was only R2900. I enquired with Standard bank and they gave ref number and said they would get back to me. They sent a message saying it was resolved. I phone standard bank to enquire and they said they did not owe me money because they checked the atm reports and it was fine. This is unacceptable, it is *******. How do I know that the machine is maintained and calibrated. I have banked with Standard since 2001, but because this type of attitude and bad service i will be changing to another bank very soon.