Items getting removed from Cache earlier than the specified expiration time in ASP.Net

Hi, I wrote this code for inserting Items in shopping cart and I expect one day Shopping Cart Items be available.

if (Cache[username] == null)
            {
                ShoppingCart cart = new ShoppingCart();
                Cache.Insert(username, cart, null, DateTime.MaxValue, TimeSpan.FromDays(1));
            }
            if (!Page.IsPostBack)
                BindGrid();
but after 30 minutes items get removed.
and I have this code in login button click event
 

if (chk_rememberme.Checked)
            {
                expiration = DateTime.Now.AddMonths(1);
            }
            else
            {
                expiration = DateTime.Now.AddMinutes(30);
            }
            FormsAuthenticationTicket AuthTicket = new FormsAuthenticationTicket(1, txt_uname.Text, DateTime.Now, expiration, false, role);

I think expiration=DateTime.Now.AddMInutes(30) in login click event affect FromDays(1). am I right? if yes what's the relation between them? how can I solve it?

..............................

and I face another problem that I think it's related to above problem. I have a cateogry of product when I click on a category I send categoryid via querystring to another page and fetch iformation about that from database and bind them in repeater this is first line of code <asp:Repeater ID="rpt_lookpd" runat="server" EnableViewState="false"> I found product doesn't show in second page but before there wasn't this problem. when I removed EnableViewState="false" It worked again. I don't know what's problem.

I asked this question in this link

http://forums.asp.net/t/1868508.aspx/1?Items+of+Shopping+cart+stroed+in+cache+get+removed+in+wrong+time

but no success.

please help me!