Mango airlines empty promises

I booked a flight with Mango airlines in November 2014. At the time the online credit card transaction froze, resulting in the funds going through but no ticket being issued. I had to buy a separate ticket for that trip and never got a refund for the initial one. I have been emailing Mango airlines, who have promised to refund the amount after they investigated with their IT department. I sent them my bank statements as requested and keep being told up to this day that they are looking into it. It has now been seven months since my refund application.