I guess you're using only the 'Simple Stock Levels' module which blocks the ordering of a product when the inventory level reaches 0. This module is very useful but it doesn't provide any reports, so I'm using the 'Stock' module in addition, so I can instantly check my stock levels and receive notifications. The 'Stock' module has the same - bug, when you edit an order or add a new one with admin account, the stock levels are not touched.