Server code can't return an object

Server code can't return an object

I am using PageMethods to call ASP.NET server code. 

My object consists of many properties type, including list object.

For example, 

Public class ClassA {
public string stringA {get;set;}
public List<ClassC> ListC {get;set;}

When the List<ClassC> has more than 1 item, the object will not be able to return to the PageMethods function

The PageMethods function is like below:
PageMethods.MyCalling(id, function(obj) {

   if(obj) <-- will not be reach ...
});

However this does not happen in my localhost. 

Why? What has been the possible problem. 

Any idea?

Thank You