Session variable set to null between web service method calls

In my web service I need to keep track of a value called store number.  I have two methods I use to attempt to do this.  Both have EnableSession=true set.

Method #1 - SetStoreNbr(string StoreNum) - I call this method and pass it the value.  The value is then stored in the Session variable, like this: 

Session["StoreNum"] = StoreNum;

 In method #2 - GetStoreNbr(), I attempt to retrieve the value from the Session variable, like so:
string str = Session["StoreNum"].ToString();
However, when GetStoreNbr() is called, the Session variable is null, resulting in the following error:  

System.Web.Services.Protocols.SoapException: Server was unable to proc ...