Unable to specify which example should be used to respond to an API

I create a sample API with two examples for a given endpoint/method with stoplight studio 1.1.5. When I send request (POST) I receive the first example as response but when I use the ?__example=xxx where xxx is the name of the example it returns a 404 message.

{
“type”: “https://stoplight.io/prism/errors#NOT_FOUND”,
“title”: “The server cannot find the requested content”,
“status”: 404,
“detail”: “Response for contentType: application/json and exampleKey: not-found does not exist.”
}