Magento – Ajax add to cart does not always work

When adding to cart, I only sometimes get the stylish Ajax "Your item was added to cart"-popup. Other times I get the ugly standard magento added to cart orange and grey buttons. When I get the default behaviour, I can't see the items in cart before refreshing the page, so I suspect it's some kind of ajax error. I have captured both Ajax requests and they are quite different. Can someone figure out the error?

Works:

$.get('http://www.propagandashop.dk/proaddto/index/add/uenc/aHR0cDovL3d3dy5wcm9wYWdhbmRhc2hvcC5kay8,/product/754/form_key/vRnRi3VEvUkNw6xa/isAjax/1')
  .done (showResult)
  .fail(showError);


  {
  "frameId": 0,
  "method": "GET",
  "parentFrameId": -1,
  "requestId": "13544",
  "tabId": 57,
  "timeStamp": 1440573329737.51,
  "type": "xmlhttprequest",
  "url": "http://www.propagandashop.dk/proaddto/index/add/uenc/aHR0cDovL3d3dy5wcm9wYWdhbmRhc2hvcC5kay8,/product/754/form_key/vRnRi3VEvUkNw6xa/isAjax/1"
}

Doesnt work:

First get:

var queryData = {
  "block[]": "options",
  "awacp": "1",
  "no_cache": "1"
};
$.get('http://www.propagandashop.dk/checkout/cart/add/uenc/aHR0cDovL3d3dy5wcm9wYWdhbmRhc2hvcC5kay9rb2trZW4,/product/235/form_key/vRnRi3VEvUkNw6xa/', queryData)
  .done (showResult)
  .fail(showError);


  {
  "frameId": 0,
  "method": "GET",
  "parentFrameId": -1,
  "requestId": "13875",
  "tabId": 57,
  "timeStamp": 1440573421999.0598,
  "type": "xmlhttprequest",
  "url": "http://www.propagandashop.dk/checkout/cart/add/uenc/aHR0cDovL3d3dy5wcm9wYWdhbmRhc2hvcC5kay9rb2trZW4,/product/235/form_key/vRnRi3VEvUkNw6xa/?block%5B%5D=options&awacp=1&no_cache=1"
}

Second get (same time):

var queryData = {
  "actionData": "{\"added_product\":\"235\"}",
  "block[]": [
    "cart",
    "sidebar",
    "topLinks",
    "skipLinks",
    "wishlist",
    "miniWishlist",
    "addProductConfirmation",
    "removeProductConfirmation"
  ],
  "awacp": "1",
  "no_cache": "1"
};
$.get('http://www.propagandashop.dk/kokken', queryData)
  .done (showResult)
  .fail(showError);

  {
  "frameId": 0,
  "method": "GET",
  "parentFrameId": -1,
  "requestId": "13876",
  "tabId": 57,
  "timeStamp": 1440573422599.8179,
  "type": "xmlhttprequest",
  "url": "http://www.propagandashop.dk/kokken?actionData=%7B%22added_product%22%3A%22235%22%7D&block%5B%5D=cart&block%5B%5D=sidebar&block%5B%5D=topLinks&block%5B%5D=skipLinks&block%5B%5D=wishlist&block%5B%5D=miniWishlist&block%5B%5D=addProductConfirmation&block%5B%5D=removeProductConfirmation&awacp=1&no_cache=1"
}