XSD element required despite minOccurs=0

Question

I am new to XSD and confused at current behavior and I am unsure what I am missing. In building a WCF service in VB I am trying to creating some elements that are optional. I have 2 elements defined that I want to be optional, either they are part of the webservice request or not, I don't care. However I keep getting errors that both are required. Am I missing something obvious? Sequence and All both should be ok with minOccurrs, as well as references.

In my research I know I am to use the minOccurs="0" As cited below from w3Schools

MinOccurs defined...

The "minOccurs" indicator specifies the minimum number of times an element can occur.

<xs:element name="person">
   <xs:complexType>
     <xs:sequence>
       <xs:element name="full_name" type="xs:string"/>
       <xs:element name="child_name" type="xs:string"
       maxOccurs="10" minOccurs="0"/>
     </xs:sequence>
   </xs:complexType>
 </xs:element> 

The example above indicates that the "child_name" element can occur a minimum of zero times and a maximum of ten times in the "person" element.

With this I have defined my ThirdPartyMessage as follows: With the goal of having ExternalID and SecondID (no control on the name)

<xs:element name="ThirdPartyMessage">
    <xs:complexType>
      <xs:all>
        <xs:element ref="Subject"/>
        <xs:element ref="Message"/>
        <xs:element ref="RequestType" />
        <xs:element ref="ExternalID" minOccurs="0"/>
        <xs:element ref="SecondID" minOccurs="0"/>
      </xs:all>
    </xs:complexType>
  </xs:element>

As well as within

  <xs:complexType name="MessageHeaderType">
    <xs:sequence>
      <xs:element name="RqUID" type="UUID"/>
      <xs:element name="AsyncRqUID" type="UUID" minOccurs="0"/>
      <xs:element name="PartnerKey" type="Identifier" minOccurs="0"/>
      <xs:element ref="ExternalID" minOccurs="0" maxOccurs="1"/>
      <xs:element ref="SecondID" minOccurs="0" maxOccurs="1"/>
    </xs:sequence>
  </xs:complexType>

Defined as

  <xs:element name="ExternalID">
    <xs:simpleType>
      <xs:restriction base="xs:string">
        <xs:minLength value="0"/>
        <xs:maxLength value="15"/>
      </xs:restriction>
    </xs:simpleType>
  </xs:element>

  <xs:element name="SecondID">
    <xs:simpleType>
      <xs:restriction base="Integer">
      </xs:restriction>
    </xs:simpleType>
  </xs:element>

Solution

Global declarations cannot contain the attributes minOccurs, maxOccurs, or use. You might want to refer this link. http://www.w3.org/TR/xmlschema-0/#OccurrenceConstraints