Access Token Passed from Web App to .NET API Not Working

I have an endpoint on my API that is decorated with the [Authorize] attribute. When I pass an Access Token from the Web App to the API App, which is obtained by the Web App after authenticating the User with Auth0, the API endpoint returns a 401 Unauthorized. When I pass a machine -to-machine Access Token (obtained from the Auth0 Test section on the API Configuration screen) to the API then it goes through Authorization fine. But that M2M token is not useful to me cause I need the user claims which are only available through the Login process.