@XmlElement(required=true) for @WebParam does not work

I'm building web service using JAX-WS. I have a strange problem that the annotation @XmlElement(required=true) for @WebParam works in some @WebService class, but doesn't work in some others.

I have very similar code in the two @WebService classes. What may cause this problem? Parameter type or the entity class?

Edit: Add sample code

I have two web services:

@WebService(name = "ClubMemberPortType", serviceName = "ClubMemberService", portName = "ClubMemberSoapPort", targetNamespace = "http://club.com/api/ws")
public class ClubMemberWS {
@WebMethod(operationName = "findClubMembersByClubId", action = "urn:findClubMembersByClubId")
    @WebResult(name = "club_membership")
    public List<ClubMembership> findClubMembershipsByClubId(@XmlElement(required=true)
                                                        @WebParam(name = "club_id") String clubId, 
                                                        @WebParam(name = "status") StatusEnum status){
...
}}

and

@WebService(name = "ClubPortType", serviceName = "ClubService", portName = "ClubSoapPort", targetNamespace = "http://club.com/api/ws")
public class ClubWS {
@WebMethod(operationName = "findClubByClubId", action = "urn:findClubByClubId")
    @WebResult(name = "club")
    public Club findClubByClubId(@XmlElement(required=true)
                                @WebParam(name = "club_id") String clubId) {
...
}}

The generated schema for the first web method is:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:ws="http://club.com/api/ws">
   <soapenv:Header/>
   <soapenv:Body>
      <ws:findClubMembersByClubId>
         <club_id>?</club_id>
         <!--Optional:-->
         <status>?</status>
      </ws:findClubMembersByClubId>
   </soa