Refuse to refund/exchange broken product

We have purchased the Melissa and Doug Sound puzzle from Toys R Us in Somerset West. From the start the product only worked on occasion. The sounds are inconsistent. It does not always go off when a puzzle piece is inserted. It also randomly goes off when no one is using it. We took it back to Toys R Us. They tested the product in store and it was working by that time. I explained that it does not always work but they insisted that it works and that it cannot be exchanged. As a customer I at least expect that the product is further investigated at the manufacturer. I would not recommend people to support Toys R Us as they have very poor customer service and returns policy.