title: JAXB fails unmarshalling some substitutionGroups
body: <p>I'm having issues unmarshalling XML that has been validated against a schema.</p>
  
  <p>I have a large schema provided to me that I have extended.  Here's a snippet of what's provided:</p>
  
  <p>(Namespace "original")</p>
  
  <pre><code>&lt;xs:element name="Platform" type="PlatformType" abstract="true" /&gt;
  
  &lt;xs:complexType name="PlatformType" abstract="true"&gt;
     ...
  &lt;/xs:complexType&gt;
  
  &lt;xs:element name="SpringPlatform" type="SpringPlatformType" substitutionGroup="Platform" /&gt;
  
  &lt;xs:complexType name="SpringPlatformType"&gt;
      &lt;xs:complexContent&gt;
          &lt;xs:extension base="PlatformType"&gt;
              ...
          &lt;/xs:extension&gt;
      &lt;/xs:complexContent&gt;
  &lt;/xs:complexType&gt;
  </code></pre>
  
  <p>Now I've extended SpringPlatformType with some additional functionality:</p>
  
  <pre><code>&lt;xsd:element name="MySpringPlatform" type="MySpringPlatformType"
          substitutionGroup="original:SpringPlatform" /&gt;
  
  &lt;xsd:complexType name="MySpringPlatformType"&gt;
      &lt;xsd:complexContent&gt;
          &lt;xsd:extension base="original:SpringPlatformType"&gt;
              &lt;xsd:sequence&gt;
                  ...
              &lt;/xsd:sequence&gt;
          &lt;/xsd:extension&gt;
      &lt;/xsd:complexContent&gt;
  &lt;/xsd:complexType&gt;
  </code></pre>
  
  <p>I am receiving a list of Platforms, like this:</p>
  
  <pre><code>&lt;xs:element name="PlatformList" type="PlatformListType" /&gt;
  
  &lt;xs:complexType name="PlatformListType"&gt;
      &lt;xs:sequence&gt;
          &lt;xs:element ref="Platform" maxOccurs="unbounded"/&gt;
      &lt;/xs:sequence&gt;
  &lt;/xs:complexType&gt;
  </code></pre>
  
  <p><strong>I can correctly unmarshall incoming messages that use the original:SpringPlatform elements, but when I try to unmarshall my derived/extended types (MySpringPlatform), I get null.</strong>  However, the XML validates against my schemas using XMLSpy 2011 just fine.</p>
  
  <p>Any suggestions?  I am using JAXB (Metro) 2.2.4 on Java 6u24, Windows XP SP3.</p>
  
  <p>Thanks,</p>
  
  <p>Brian</p>