Faulty brand new microwave from HI Fi Corporation

Hi I bought a new microwave by Hi Fi Corporation on the 29th July 2022. When we started using the new microwave we noticed that it was not heating as it should. On certain days it will take up to 15 minutes to get a plate of food warmed up and on other days after 3 minutes then the food would be warm. Then I took the microwave back to the store on 14 August 2022 and told them the microwave is faulty.They tested it by warming up water on three occasions and the water got warm as it should. They said the microwave is fine and they couldn't exchange it or refund me because the microwave was working fine when they tested it. When I got home I tested it again and it worked but the evening it was again not working. Yesterday afternoon it was again not heating up but yesterday evening it was heating up as it should. Its disappointing to have spent money on a new microwave and having to struggle with something that should be working properly. I would like feedback on this problem as it's very frustrating. I can be contacted on *** or ***