Taking clients for a fool.

My credit card was due on the 31st and a debit order of R334 would be deducted as per the statement. On the 24th so I knew everything was done I paid the balance less the R334. Then FNB didn't do the debit so that means R334 was outstanding to them so then they charged R211.77 in interest. I don't know how I thus can be the case. I'm being taken for a fool because I paid my card in full and FNB didn't debit their portion.