Assume that  your *form action* url  is `checkout/cart/updatePost/` and  have no customization at `Mage_Checkout_CartController` at **updatePostAction()**.

Then you need to understand  that magento `checkout/cart/updatePost` does ***not render Cart page*** content.so if **you have try to get
 cart page**  content from   **checkout/cart/updatePost/**  then it become waste of time.

 Because of   `checkout/cart/updatePost` is response 302 after done anything.So your case you need to customization at cart controller

 Also,as you have use magento 1.9  then you need to check **form_key  field  is available or not in  that  form**