Unhappy with FNB feedback with regards to a credit payment on my account

On the 27th June I've paid R900 towards my FNB Credit Card as a payment of R1216-00 was due for my installment. Another credit payment was done on the 9th June 2018 with an amount of R434-40 on my credit cards which gives me a total credit towards my credit card of R1334-40. Today I was contacted by FNB telling me that I still need to pay R316-00 that's outstanding on my credit card because the credit payment of R434-40 on the 9th June can not be taken into consideration for payment. I want to know why it can not be taken as a payment because the call center agents phoning we could not give me any answers as well as the people I had to phone on ********** 846 could also not tell me why. According to me it's a payment and that must be taken into consideration as a credit payment. Please explain??