Session Variables

I have a website I wrote where I store information that I need from page to page in session variables. I set the variable in the global.aspx to "". Then in my code I simply reference it like: Session("CheckType") = "DIRECT"

Now would anyone know why this would sometimes not keep? Some of my session variables work fine and keep their values from page to page. There is one however where I can assign a value on one page for it and it never keeps, so when I reference the value on another page it doesn't work. Ideas?

Thanks!It won't stay in memory if the session times out. Is that what's happening?