stock changes after sale

i update my webshop product through the api, based on the last modification date
i get the stock values by ["Item"]["ItemShops"]["ItemShop"][0]["qoh"];
but when there is an in-store sale, I think a sale does not change the last modificated date of the product
and the product is not in the requested feed
so i do not get an update on the stock of this product with the api
any idea what i'm doing wrong?
or is it nescessary to use another request for a list of sales and stockchanges