Why do they ghost customers after accepting our bookings and money?

I purchased tickets to Chris Brown then wanted to upgrade. Their online support said I must buy another set of tickets and I will be refunded for the original 2. I did this and they are now quiet. No call centre. No response on email. No response on Twitter. The only thing I can do now is approach my bank for a chargeback as I have it on mail that I would be refunded but they are ghosting me and hundreds of other people.