[SalesForce] HttpRequest and HttpResponse is not covering in test class

I wrote a Testclass(Mock response) by implementing HttpCalloutMock interface to cover HttpRequest in class, I given some fake values to it also. But while calling the class, its not covering in regular test class.

Following is the class to cover http request and response:

Http h = new Http();
HttpRequest req1 = new HttpRequest();
req1.setMethod('POST');
req1.setHeader('Content-Type', 'application/json');
req1.setHeader('Authorization', authorizationHeader);
req1.setbody(reqbody);
req1.setTimeout(120000); 
req1.setEndpoint(endpoint);

HttpResponse res1 = new HTTPResponse();
res1 = h.send(req1);

But, The above class will return string. For the above class i wrote the following mock response test class:

@isTest
global class MockGenerator implements HttpCalloutMock {

    global HTTPResponse respond(HTTPRequest req) {

        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setHeader('Authorization', 'Basic YXBmFtaWx5');
        res.setBody('{"Username":"testuser@test.com", "owner":"testowner@test.com", "uniqueid":"123456789", "email":"testemail@test.com"}');
        res.setStatusCode(200);
        return res;
    }
}

I called the mock response in main test class even, but its not covering. I tried a lot to get cover and always getting fail. DO i am missing any thing. Please guide in this.

Test.setMock(HttpCalloutMock.class, new MockGenerator());
String response = MyRewardAPI.Authenticateuser();

Above one is calling mock test in main test.
Thank you