Unexplained Cash Finance Charges on Credit Card

I have contacted Standard Bank several times over the past few years to under how the cash finance charge on my credit card is calculated but not one person has provided a response that makes logical sense. I am now paying approximately R800 per month over and above the R2500 payment per month and the last payment deducted was R5000 and still included a cash finance charge. The amount used on the card has been less than 6 months so I am failing to understand these charges. Please can someone who understands the calculation, please contact me.