App return rejected

I bought video editor app LumaFusion for £30, I tried editing a video but it doesn’t work, I contacted their support and sent a sample video I’m trying to work with, they said they can reproduce the issue but don’t know why it doesn’t work and suggest encoding the video on PC with a different codec. I requested a refund next day via the website and it was rejected. I went on Apple chat support and they said because I already requested a refund and it was rejected they cannot change that decision and there’s nothing they can do. I’m stuck with a useless app and no refund… Any ideas what can I do to get a refund?

Update: Apple doesn't give refunds for app purchases, even my bank couldn't help despite the fact I used a credit card.