WCF not exposing method - wcf

Question

I have a service contract defined as:
[System.ServiceModel.ServiceContractAttribute(Namespace = "http://www.ans.gov.br/tiss/ws/tipos/tisscancelaguia/v30001", ConfigurationName = "ItissCancelaGuia")]
public interface ItissCancelaGuia
{
[System.ServiceModel.OperationContractAttribute(Action = "tissCancelaGuia", ReplyAction = "*")]
[System.ServiceModel.FaultContractAttribute(typeof(tissFaultWS), Action = "", Name = "tissFaultWS", Namespace = "http://www.ans.gov.br/padroes/tiss/schemas")]
[System.ServiceModel.XmlSerializerFormatAttribute(SupportFaults = true)]
[System.ServiceModel.ServiceKnownTypeAttribute(typeof(SignatureType))]
mensagemTISS tissCancelaGuia_Operation(string mensagem);
}
And the implementation:
[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Single, ConfigurationName = "CancelaGuiaTS")]
public sealed class tissCancelaGuia : ItissCancelaGuia
{...}
and also the relevant parts of the web.config:
<service behaviorConfiguration="tissCancelaGuiaStringServiceBehavior"
name="CancelaGuiaTS">
<endpoint address="" behaviorConfiguration="EndPointValidation"
binding="basicHttpBinding" bindingConfiguration="TISSv3_binding"
name="CancelaGuiaTs" contract="ItissCancelaGuia" />
<endpoint address="mex" binding="mexHttpBinding" name="CancelaGuiaTsMetadata"
contract="IMetadataExchange" />
</service>
and
<behavior name="tissCancelaGuiaStringServiceBehavior">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="true" />
</behavior>
The service compiles correctly but the wsdl exposed contains no operations.
The only solution I can think of is to write the wsdl manually and expose it through the externalMetadataLocation attibute of the serviceMetadata, but it just don't feel right to me.
Edit 1
Here the definition of the mensagemTISS class
[System.CodeDom.Compiler.GeneratedCodeAttribute("System.Xml", "4.0.30319.1")]
[System.SerializableAttribute()]
[System.Diagnostics.DebuggerStepThroughAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true, Namespace = "http://www.ans.gov.br/padroes/tiss/schemas")]
public partial class mensagemTISS
{ ... }
Don't know if it is important but the class have this property:
[System.Xml.Serialization.XmlElementAttribute("operadoraParaPrestador", typeof(operadoraPrestador), Order=1)]
[System.Xml.Serialization.XmlElementAttribute("prestadorParaOperadora", typeof(prestadorOperadora), Order=1)]
public object Item
{
get
{
return this.itemField;
}
set
{
this.itemField = value;
}
}
defined as a choice in the originating schema.