bad customer services at GAME festival mall

I went to game store in Kempton park festival mall, I bought a Russel Hobbs pressure cooker on the 7th December 2019. I went home and opened my pressure cooker and used it. it was slow and I put it back in the box to take it back to the shop, I went on the 9th December 2019. I told them the pressure cooker is slow and they said they cant take it back because I have already used it. they called someone who deals with the department and she said she also have the same pressure cooker and it works perfectly. then they called the manager. she came to assist she was speaking in her language when I respond in my language she said she doesn't hear me I should speak something that she will understand. I told her the Tsonga Language is official in SA. she the told me I should use business language in a very rude way. she said to the cashier give her money back and she left. it was very bad customer service that I received.