Whenever an order has been placed causing a change in inventory, or your inventory changes in your storefront, a string of updates is queued up in the Shoppingfeed server. Depending on server load and the number of products you sell, these updates could take up to 5 hours to complete.
When the changes to your inventory are made on your storefront, to see updates in all channels and in Shoppingfeed immediately, you need to update your Source Feed.