Shocking Service From FNB

On 6 May FNB Credit Card debited my account with interest by mistake due to a system failure, on Monday 10 May I queried this and was given a reference ***. On Tuesday 18 May I queried why I had received no response, and given another reference *** Same day I received an email why stated "We have reversed the interest that has been charged on your credit card due to the system error that we have experienced " On Friday 21 May I queried WHY the interest had not been reversed, I received no response. On Saturday 22 May the interest has still not been reversed. Is FNB in the habit of lying to their customers, or is it just a case of Gross Incompetence?