No answers from FNB

I have always been a huge fan of FNB but they have disappointed me of late. I am a Platinum card&account holder. They keep on charging interest on my credit card and I do not know what for. I do not exceed my credit card limit, I always repay the full amount on time, I do not purchase fuel on my credit card neither do I draw cash using my credit card. I have been in contact with more than 3 people on 3 separate occasions and I have been to a branch, but nobody can help me to clear this interest. They all say "it must be from doing x or y" (as mentioned above), but it is not, as I do not do x and y. I have had an FNB account/credit card for over 6 years and never incurred this interest. I really do need some feedback - on email, not via a phone call as I consult during the day.