multiple ajax forms in the same mvc 4 view doesn't work properly - after the second time of submit my model is empty - asp.net-mvc-4

Question

I've got a class Cart with property Lines which returns me a collection of type CartLine. here is my two classes Cart ad CartLine
public class Cart
{
private List<CartLine> lineCollection = new List<CartLine>();
public String Currency { get; set; }
public void UpdateCart(int ArticleID, string selectedQuantityType, int Quantity)
{
CartLine line = lineCollection.Where(p => p.Product.ArticleID == ArticleID).FirstOrDefault();
if (line != null)
{
line.SelectedQuantityType = selectedQuantityType;
line.Quantity = Quantity;
}
}
public IEnumerable<CartLine> Lines
{
get { return lineCollection; }
}
}
<
And here is my class CartLine
public class CartLine
{
public CartLine()
{
this.QuantityType= new List<QuantityType>();
}
public WebServiceBeaMenu Product { get; set; }
public int Quantity { get; set; }
public string Currency { get; set; }
public string SelectedQuantityType { get; set; }
}
I've got two actions which returns me strongly typed PartialView - CartIndexViewModel with two properties - one from Type Cart
public ActionResult CartPartial(string returnUrl)
{
return PartialView(new CartIndexViewModel
{
Cart = GetCart(),
ReturnUrl = returnUrl
});
}
[HttpPost]
public ActionResult CartPartial(CartLine line)
{
Cart crt = GetCart();
crt.UpdateCart(line.Product.ArticleID, line.SelectedQuantityType, line.Quantity);
return PartialView(new CartIndexViewModel
{
Cart = crt
});
}
private Cart GetCart()
{
Cart cart = (Cart)Session["Cart"];
if (cart == null)
{
cart = new Cart();
Session["Cart"] = cart;
}
return cart;
}
ANd here is my view whih loop through all thre cartlines of my cart and display their quantity and quantityType.
The problem is as you see for each CartLine I'm making an ajax form so when there is a change in the quantity of a CartLine (when there is a change in this helper #Html.TextBoxFor(x => line.Quantity,new { id = #liine.Product.ArticleID, onchange="SubmitForm(this.id)" })) you see that I submit the form to the httppost form of my CartPartial Action. The first time when I submit the form there is no problem and thanks to mvc model binding my CartLine parameter is filled with the data I need. The problem is when this action returns me the same view with the updated data after that if I try to change a quantity and submit the form for the second time my CartLine parameter is empty. What may be the reason. Thankls in advance.
#model MvcBeaWeb.Models.CartIndexViewModel
<div id="cartContainer">
<table width="100%" align="center" style="margin:auto;border-collapse:collapse;">
<tbody>
#foreach(var line in Model.Cart.Lines)
{
<tr style="height:90px" >
#using (Ajax.BeginForm("CartPartial", null, new AjaxOptions { UpdateTargetId = "centerCartBody", InsertionMode = InsertionMode.Replace }, new { id = "form" + #line.Product.ArticleID }))
{
<td align="left">#line.Product.SpecialWord</td>
<td >
<div class="inner-content">
#Html.TextBoxFor(x => line.Quantity,new { id = #liine.Product.ArticleID, onchange="SubmitForm(this.id)" })
<span class="qty-type">
#Html.DropDownListFor(x => line.SelectedQuantityType, new SelectList(line.QuantityType, "QuantityID", "Description"))
</span>
</div>
</td>
#Html.HiddenFor(x => line.Product.ArticleID, "Value");
}
</tr>
}
</tbody>
</table>
<div >
#using (Html.BeginForm("Index/","Confirmation"))
{
<input type="submit" id="btnCheckOut" />
}
</div>
</div>
</div>
</div>
script>
function SubmitForm(id)
{
$("#form_" + id).submit();
}
</script>