When traders attempt to sell short a stock which IBKR does not currently have in inventory to loan them, IBKR will look for these shares “on the street”, which means from other brokerage firms.  This search is conducted on a best-efforts only basis.  While IBKR searches for the shares, the order status box on the TWS Order Management page should be dark green and will show a small icon of a pair of binoculars, which indicates we are searching.  In the WebTrader, there are no status colors or icons.  The order will simply not execute as IBKR searches for the shares on the street.