The mini-cart is only going to work at exactly the same domain that the shopping cart is at. In your case, your shopping cart is at the domain
http://yourdomain.com. However, all your store pages you are forcing to load at the domain
http://www.yourdomain.com (for example, try going to
http://yourdomain.com, you will note that you are immediately redirected to
http://www.yourdomain.com).
You can either change this so you force all store pages to just the domain
http://yourdomain.com (this is a server/site configuration option, not a ShopSite option), or you can change your ShopSite Shopping Cart URL (set in Preferences -> Hosting Service) and all of your order and view cart buttons to use the domain
http://www.yourdomain.com so that the store pages at that domain can utilize the mini-cart. Either way, you need to use the same exact domain in both places.
-Loren