30 day return policy not honoured by them

I bought Ecovacs Deebot N8 PRO+ Robot Vacuum Cleaner. Their policy says to return within 30 days if you have ch aged your mind. My first return I told them the vacuum does not pick up dust at all hence I’ve changed my mind on this particular vacuum. Each time they sending it back to me saying it’s not defective. I don’t want the vacuum because it does serve my need and they refuse to accept it and keep saying it’s not defective. But that’s not the point. It’s not serving my need and I returned it within the exact 30 day time period which they show on their term. I’m allowed to change my mind. But they refusing to allow me to change my mind.