奇怪，为什么ModuleLoader会一直在ModuleEvent.PROGRESS中跳不出来,该如何处理

<?xml version="1.0" encoding="utf-8"?>
<mx:Module xmlns:mx="http://www.adobe.com/2006/mxml" layout="vertical" width="100%" height="100%"
           xmlns:ns3="modules.*"
           creationComplete="init()">   
    <mx:Script>
    <![CDATA[
    import mx.rpc.events.FaultEvent;
        import mx.controls.Alert;
        import mx.modules.IModuleInfo;
        import mx.events.ModuleEvent;
        import mx.modules.ModuleLoader;
        import mx.modules.ModuleManager;

        private function init():void
        {
            initUserManagerModuleLoader.url = "modules/UserManager.swf";
            contentViewStack.percentWidth = 100;
            contentViewStack.percentHeight = 100;
            contentViewStack.selectedChild = initUserManagerModuleLoader;
        initUserManagerModuleLoader.addEventListener(ModuleEvent.ERROR, initModuleError);
            initUserManagerModuleLoader.addEventListener(ModuleEvent.READY, initModuleReady);
            initUserManagerModuleLoader.addEventListener(ModuleEvent.PROGRESS, initModuleProgress);
        }

    private function initModuleError(event:FaultEvent):void
    {
        Alert.show(event.message + "");
    }

        private function initModuleReady(moduleEvent:ModuleEvent):void
        {
            Alert.show("initModuleReady");
        }

        private function initModuleProgress(moduleEvent:ModuleEvent):void
        {
       Alert.show("initModuleProgress");
        }
            
        ]]>
    </mx:Script>    
    <mx:HBox width="100%" height="100%" horizontalGap="0">
        <mx:Panel width="15%" height="100%" layout="absolute" textAlign="center">
            <mx:Accordion headerStyleName="accordion" width="100%" height="100%">               
                <mx:Canvas label="系统管理"
                           width="100%"
                           height="100%">
                </mx:Canvas>
            </mx:Accordion>
        </mx:Panel>
        <mx:ViewStack id="contentViewStack">
            <mx:ModuleLoader id="initUserManagerModuleLoader" width="85%" height="100%"/>
            <mx:ModuleLoader id="registerUserModuleLoader" width="85%" height="100%"/>
        </mx:ViewStack>
    </mx:HBox>

</mx:Module>

<?xml version="1.0" encoding="utf-8"?>
<mx:ModuleLoader xmlns:mx="http://www.adobe.com/2006/mxml" width="589" height="483">
    <mx:Panel width="100%" height="100%" layout="absolute" title="123123" fontSize="12">
        <mx:Button x="498" y="286" label="Button"/>
        <mx:ComboBox x="231" y="425"></mx:ComboBox>
        <mx:AdvancedDataGrid x="489" y="318" id="adg1" designViewDataType="tree">
            <mx:columns>
                <mx:AdvancedDataGridColumn headerText="Column 1" dataField="col1"/>
                <mx:AdvancedDataGridColumn headerText="Column 2" dataField="col2"/>
                <mx:AdvancedDataGridColumn headerText="Column 3" dataField="col3"/>
            </mx:columns>
        </mx:AdvancedDataGrid>
    </mx:Panel>    
</mx:ModuleLoader>