On certain vehicles, some FFV positions can't be locked with lockTurret immediately after the vehicle is created. This applies to e.g. the front cargo bay seats in the Offroad. lockTurret does work if a delay is introduced between vehicle creation and the execution of the command. (Mission start counts as vehicle creation for preplaced vehicles; this affects the Editor init field for vehicles.)
This problem appears to be exclusive to FFV positions where the option to turn out and turn in is available. For normal turrets and FFV positions, no delay is required.