Do you have sport mode on? I know in my car (X350 XJR) the car basically refuses to change into 6th gear when cruising on the freeway if I have sport mode on. This becomes even more true when I have just kicked-down to pass a car or get up to speed.

An easy way to get the car into 6th is to press the sport mode button to turn it off. Once you do this, the car will immediately change into 6th gear. After this happens, just press the sport mode button again to turn it on. The car should still be in 6th.

Hope this helped.

 

Thats interesting. My 3.0 S-type has the same issue. I always have to turn the sport button off to hit high gear.

 

That is the proper operation of the ZF. While in sport mode, it will not enter the overdrive 6th gear. I wouldn't mess with the sport button by turning it off and on to engage overdrive. The ZF is also one of many modern adaptive transmissions, meaning it is supposed to learn and adapt to the way it is driven. When the input is inconsistent it gets confused and produces some of the harsh issues that people complain about. I drive my 3.0 pretty hard and it not runs through the gears on acceleration as if sport mode is on yet I hardly ever engage the button manually.