Payment not credited to account

I made a payment on my garage card of R1500 on the 26th of May. to date, they say they cannot find or trace it. I requested this information on Monday, to date I am told that this is not resolved. How long does it take top trace a payment. Two payments were made. On was traced but the one made earlier for a larger amount cannot be traced. I am not going to pay interest charges for incompetence!!