How to Increase session memory on iis server

How to Increase session memory on iis server

I have a asp.net project who hosted on iis7.5 server where number of user
is register simultaneously (Approx 2 user in a minutes) and after
submitting the form i have saved user id (which is unique) in session. I
will treated this user id as invoice no for the payment gateway. but here
is trouble! the payment gateway will deducted the amount and redirecting
to my thanks page but it will not updated the data of my thanks page
because it is giving error sometime. Also the bank has provided us the
pages where they received responded data through below chunk of code
Session["MerchTxnRef"] =
Page.Request.QueryString["vpc_MerchTxnRef"].Length > 0 ?
Page.Request.QueryString["vpc_MerchTxnRef"] : "Unknown";
Session["AuthorizeId"] =
Page.Request.QueryString["vpc_AuthorizeId"].Length > 0 ?
Page.Request.QueryString["vpc_AuthorizeId"] : "Unknown";
Session["ReceiptNo"] = Page.Request.QueryString["vpc_ReceiptNo"].Length >
0 ? Page.Request.QueryString["vpc_ReceiptNo"] : "Unknown";
Session["Amount"] = Page.Request.QueryString["vpc_Amount"].Length > 0 ?
Page.Request.QueryString["vpc_Amount"] : "Unknown"; Session["CardType"] =
Page.Request.QueryString["vpc_Card"].Length > 0 ?
Page.Request.QueryString["vpc_Card"] : "Unknown";
Please observer that there is 5-6 session variable using for one user,
some of created before submitting and some are using after the payment
deduction.
my problem is that some time this page(which is provided by the bank)
didn't collect the responded data and it will give the object reference
error. It is very strange that it is happening for few users.The approx
average is 10 out of hundred 100.
My assumption is there is heavy load on IIS server and session didn't work
properly. So 1. Can i increase the session memory so that it will work if
yes then how? 2. Should i call to our hosting provider?
Please response