Onchange event with adobe air and javascript

I'm developing an application with adobe air and javascript. I have a select element where I added an onchange event . Every time the select change value the application make a ajax call to fetch data to populate another field. But I can not get any response from the server. Simply nothing happens. Anyone know any limitations to the event and how to overcome it?

select_object.addEventListener("change",callServer,false);  
function callServer(event){  
     var a= event.currentTarget.value;  
     var variables = new air.URLVariables("value=" + a);  
     var u ="http://www.exemple.com";   
     var request = new air.URLRequest(u); 
     request.data = variables; 
     request.method = air.URLRequestMethod.POST;  
     loader = new air.URLLoader(); 
     loader.dataFormat = air.URLLoaderDataFormat.TEXT; 
     loader.addEventListener(air.Event.COMPLETE, comp); 
     loader.addEventListener(air.Event.SECURITY_ERROR, function(event){alert('error');});   
     
     function comp(e) { alert('sucess') }  
}