Charging higher interest than that on my contract

I got my monthly statement and realised that fnb is charging me a higher interest rate than what I signed for. This is the second time in 7 months they do this. I sent an email, phoned their card division and to date I still don't have feedback. No one has contacted me nor have the corrected the interest rate. All they can tell me is that they don't have a copy of my quotation on file.