When starting, flying manually for some time (maybe half a minute or minute) and trying to make the autopilot follow the programed flight route it will in 90% of the time don´t do that - because it cannot find the SID anymore nor the first waypoint 
You press DIR (for direct to → ) and enter the letters of the first waypoint you see on the navigation display in front of the Airbus somewhere (while having the WPT button on the autopilot panel active to make the waypoints visible).
If you press heading managed on the autopilot now it will steer direct to the entered and activated waypoint, and continue to follow the complete flight path now like it should.