Why my plugin can't work correctly?

I’ve created a plugin follow the plugin develope guide.But it can’t work correctly.Then i searched the source and found that when the starter calls the loadplugin() method,it analyzes the plugin.xml of plugins in the AdminConsole and recursively puts the tab\sidebar\item into the generatedModel with the overrideTab(),overrideSidebar() and overrideEntry methods,but when it resolve my plugin.xml,it stop before the point of overrideTab().It can’t find the tab in my plugin.xml by calling getElementByID(String id) method even the tab had been added.Can anyone help me?I wish your help,thanks.

the follow is the code of my plugin.xml

<?xml version="1.0" encoding="UTF-8"?> org.wst.sample.plugin.SamplePlugin
<!-- Plugin meta-data -->
<name>SamplePlugin</name>
<description>a SamplePlugin</description>
<author>Jive Software</author>
<version>1.0.3</version>
<date>8/27/2008</date>
<minServerVersion>3.6.0</minServerVersion>
<adminconsole>
    <tab id="mytab_" name="Example" url="sampleplugin.jsp">
        <sidebar id="mysidebar_" name="SamplePlugin">
            <item id="myitem_" name="test1"
                  url="sampleplugin.jsp"
                  description="for test"/>
            <item id="myitem_2" name="test_2"
                  url="sampleplugin.jsp"
                  description="for test">
          <sidebar id="mysidebar_3" name="SamplePlugin">
              <item id="myitem_3" name="test_1"
                    url="sampleplugin.jsp"
                    description="for test"/>
              <item id="my_item_4" name="test_2"
                    url="sampleplugin.jsp"
                    description="for test"/>
          </sidebar>
       </item>
        </sidebar>
    </tab>
</adminconsole>

the follow picture shows the result page of my plugin.