That shouldn't occur if you create a driving session using the Session editor (Surveyor in Session editing mode). This will ensure that the command is always in the session if you restart it again from scratch. If you are using UDS, then there's a possibility that the driver has already executed the command when you've edited and returned to the session again.
The command works by executing and disappearing just like any other command. I used it in the past to keep my coal drags from going 70 mph on my mainline by placing the command as the first one after I've had a consist exit a portal, waited 20 seconds, loaded up engines with fuel using instant load locos, and instant load commodities before driving off using the Copy Commands from... Schedule Library command.