Boeing 787 altitude control

OK, so I have no problems with flying the A320 and spend most of the flight on autopilot. With the A320 I set the managed altitude to 12,000ft, say, and the ascent rate to 2000ft/min and it climbs to that altitude fine. I normally have managed speed selected so it climbs at 250knots up to 10,000ft and then over 300knots after that all good. So when I then want to go to cruise at 18,000ft from a stable 12,000ft then I change the managed altitude to 18,000 an reapply an ascent rate of 2000ft and all is fine.

Now with the 787 the situation seems different. You don't seem to be able to select managed speed (inop) but it is not too difficult to set a speed, fine. I also had no problem getting to 12,000ft similarly to the A320. However, at that altitude if I wanted to change to 18,000 then I select 18,000 as the managed altitude and 2000ft/min as climb rate BUT NOTHING HAPPENS. If I BRIEFLY turn on vnav then the plane starts to climb at a massive rate, I turn off VNAV but then I can engage managed altitude and climb rate again and the plane then ascends at the managed rates to the manged altitude. But WHY?? The same is true in reverse. I seem to need an on/off of VNAV for the managed descent to kick in.

I am missing something here?

Thanks

CJ