Product option selection will postback and reset options

I copied the following transformation from the demo e-commerce site to my own site:

<%@ Register Src="~/CMSModules/Ecommerce/Controls/ProductOptions/ShoppingCartItemSelector.ascx" TagName="CartItemSelector" TagPrefix="uc1" %>
<div class="productDetail">
  <p>    
    <%# Localize(Eval("SKUShortDescription")) %>
  </p>
  <div class="productInfo whiteBox">
    <table>      
      <% // Show pricing details according to saving %>
      <% if(GetSKUPriceSaving() > 0){ %>
      <tr>
          <td class="label">
             <span class="retailPrice">List price: </span>
          </td>
          <td>
              <span class="lineThrough"><%# GetSKUFormattedOriginalPrice() %></span>
          </td>
       </tr>
       <tr>
          <td class="label">
               <span class="yourPrice">Your price: </span>
          </td>
          <td>
               <span><%# GetSKUFormattedPrice() %></span>
          </td>
       </tr>
       <tr>
          <td class="label">
              <span class="instantSavings">You save: </span>
          </td>
          <td>
              <span><%# GetSKUFormattedPriceSaving() + " (" + GetSKUPriceSaving(true) + "%)"  %></span>
          </td>
        </tr>
      <% } %>

   </table>

   <uc1:CartItemSelector id="addItem" runat="server" SKUID='<%# EvalInteger("SKUID") %>' SKUEnabled='<%# EvalBool("SKUEnabled") %>' AddToCartTooltip="Add to cart" AddToCartLinkText="Add to cart" ShowWishlistLink="true" AddToWishlistLinkText="Add to wishlist" ShowUnitsTextBox="true" AlwaysShowTotalPrice="true" TotalPriceLabel="Total price:" ShowProductOptions="true" StockVisible="true" UnavailableVariantInfoEnabled="false" UsedInProductDetail="true" CssClassNormal="normal" CssClassFade="fade" />
  </div>   
</div>

I have a product with a few product options (attributes). When I select one of the options in the webshop, it will cause a postback to the server, but the selected option will return to the default one.

How do I fix this?