InstanceContextMode = InstanceContextMode.PerSession not working

Hello,
 I have simple wcf service hosted in IIS 7. The first call to the service (GetData) creates the Transaction object. I expected that the 2nd call to the service (GetData2) would have the Transaction object available. However the value of MyTrans is null on the 2nd call.   I thought that     [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]  would result in this object being retained for the session. Any idea what I am doing wrong ?

 regards
 Pat

Service code:
    [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
    public class Service1 : IService1
     {
         Transaction MyTrans;
         public Transaction GetData(string value)
         {
             MyTrans = new Transaction();
             MyTrans.companyId="CCS Ltd";
             MyTrans.trans = "Initmenu01";
             MyTrans.userId = "PClarkeirl";
             MyTrans.curSeq = 123;
             return MyTrans;
         }

         public Transaction GetData2(string value)
         {
             MyTrans.nextSeq = MyTrans.nextSeq+1;
             return MyTrans;
         }


 Client code:
         {

                 ServiceReference2.Service1Client client = new ServiceReference2.Service1Client();
                 ServiceReference2.Transaction MyClientTrans = client.GetData("XXX");
                 MyClientTrans = client.GetData2("XXX");
         }



web.config source:
 <?xml version="1.0" encoding="utf-8" ?>
 <configuration>

   <appSettings>
     <add key="aspnet:UseTaskFriendlySynchronizationContext" value="true" />
   </appSettings>
   <system.web>
     <compilation debug="true" />
   </system.web>
   <!-- When deploying the service library project, the content of the config file must be added to the host's
   app.config file. System.Configuration does not support config files for libraries. -->
   <system.serviceModel>
     <services>
       <service name="WcfServiceLibrary7.Service1">
         <host>
           <baseAddresses>
             <add baseAddress = "http://localhost:8733/Design_Time_Addresses/WcfServiceLibrary7/Service1/" />
           </baseAddresses>
         </host>
         <!-- Service Endpoints -->
         <!-- Unless fully qualified, address is relative to base address supplied above -->
         <endpoint address="" binding="basicHttpBinding" contract="WcfServiceLibrary7.IService1">
           <!--
              Upon deployment, the following identity element should be removed or replaced to reflect the
               identity under which the deployed service runs.  If removed, WCF will infer an appropriate identity
               automatically.
           -->
           <identity>
             <dns value="localhost"/>
           </identity>
         </endpoint>
         <!-- Metadata Endpoints -->
         <!-- The Metadata Exchange endpoint is used by the service to describe itself to clients. -->
         <!-- This endpoint does not use a secure binding and should be secured or removed before deployment -->
         <endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange"/>
       </service>
     </services>
     <behaviors>
       <serviceBehaviors>
         <behavior>
           <!-- To avoid disclosing metadata information,
           set the values below to false before deployment -->
           <serviceMetadata httpGetEnabled="True" httpsGetEnabled="True"/>
           <!-- To receive exception details in faults for debugging purposes,
           set the value below to true.  Set to false before deployment
           to avoid disclosing exception information -->
           <serviceDebug includeExceptionDetailInFaults="False" />
         </behavior>
       </serviceBehaviors>
     </behaviors>
   </system.serviceModel>

 </configuration>