Why would a stop fail to execute on IB Book Trader? I know it wasn't a communication failure because the prices were updating. It was a regular stop, not a limit stop with the last price as a trigger. But even though the stop was hit, it didn't trigger. I tried closing TWS and logging back on. It didn't show a stop order, but it did show a current position. Why did TWS lose the stop order? Since then I haven't had any problems, but knowing your stops might be ignored takes away a lot of the peace of mind having stops in place gives you.