Manual flying the 747X during takeoff

Most of the time I like to fly the 747 during takeoff and the first 50Nm or so and then I select Vnav and later also Lnav.I did this many times and it always worked but on my last flight I selected only Vnav and the autopilot and I could not control the plane horizontally anymore. So Lnav was not selected, only Vnav.I must have changed something but I can not find out what.Joystick is working perfect so that is not the problem.