It will not work if you also have product/orderline discounts that apply to a product (opposed to an order). Then the product discount will not be applied. You also have the "Only apply to products without a discount" option you can use. If you have i.e. 3 products in the cart and one has a discount already. Then you can add 10% on the order, but only given to the 2 products with no discount already.
It is a bit tricky - lots of settings. But hope this clarifies.