over charging of intrests on customer accounts

I would like to express with great corncern how edcon group handles their credit accounts records.it seems like when you pay accounts with them they dont deduct the amount you will have paid.i paid an amount of 2000 rands for two consecutive months but the amount i owed still remained same after the third month.it was like i had only paid 1000 rands.i am not sure the way they handle their accounts but i suggest they have to be investigated because they are overcharging clients with accounts.i also have an account with other stores and when i pay my instalment with the other stores i can see for sure that the credit is decreasing but the one for edgars even if i overpay it doesnt seem to be decreasing.please help me by finding out how they add their intrest to the accounts and how they calculate them.i have also tried to ask other people with accounts there and they are having a similar problem