Credit Card

For more than a month now (from 12th Oct) I have been trying to get FNB to tell me why am being charged interest on a credit card that should have interest free transaction for 55 days with little success. I have sent emails and spoken numerous times to the private banker and the Private Clients line but all I get is generic statements. One customer services rep told me that my query would be answered in 50 days. All this while they continue to charge interest on transactions not FNB nor the client understands. All I've asked for is to be told how they come up the interest charged on my statement. Is that to much to ask?