Struggling to get my refund

I bought a Ryobi Generator at Builders Warehouse, when I unboxed it and followed the instructions to the tee, the generator did not function as it should. I returned the generator to the store and the Ryobi sales rep was called to examine the generator. He claimed that I mixed the oil and the petrol and that's what caused the generator not to perform. Unfortunately this is both incorrect and irrational as I followed the instructions and know where to throw in the oil and where to throw in the petrol. I then requested a refund as I was sold a faulty product and to date I still have not received my refund. The product was a out of box failure and they tried to blame it on me. I have driven to the store on multiple occasions and have been put through many hurdles. I cannot deal with this anymore and have been in contact with the call centre that put me through the wring store. They are happy to take my money for the purchase but have many hurdles to refund me for their faulty products.