Session got null (some time) when call to Web services

In my web service I need to get one value from session variable. In the constructor of my web service I am getting this value like

constructor()  // constructor of web service
{
  Person p = (Person)Session["xyz"];
  string name = p.Name;
  string Address = p.Address
}

my java script is like bellow

function CallWebservice()
{
MyWebservice.GetValueFromDb(onsucess,onfailure)
}

It works perfectly fine but some time i got the session as null, i have the session state in web.config which has default time out as 60 minutes.

but before 60 minutes(not all the time) the session is getting null, this things happens only when i am calling the webservice from java script

Tags: Javascript, ASP.NET, Webservice