Currently handling overloads method is a known limitation of the Isolator framework. In your case when you set up an expectation on MethodName the first call (the one without the parameters) is caught and mocked.
A simple work around for this would be:
Mock mockAllMyClass = MockManager.MockAll(typeof(MyClass));
We are aware this is not nice but still it should make the job done (at least until we implement the full support for overload methods)