Look at your code:
Session[BloodGroup1 + "TotalUnit1"] = TotalUnit1;
And
if (Session["A+_TotalUnit"] != null)
{
Label3.Text = " " + Session["A+_TotalUnit"].ToString();
}
The index names aren't the same - the "Set the value" code ends with a "1", the "Read the value" code doesn't.