Query on the balance

I have an account with truworths for over 10 years now,now in July I paid what was owing for the goods purchased which had a 6 months plan then had cellphone which also is a 6 months plan, my query is that I have paid as per balance owing on clothing then later on the month paid for cellphone now the statement for August is still showing the installment for previous month end which didn't reduce the installment. did call customer care service but couldn't get answers went in store the cashier also couldn't help