Help with planer thicknesser

[locket&beard]

Hi everyone. I've recently reset the knives on my planer thicknesser. Now when I surface plane on the top it's perfectly flat. But when I then pass it through the thicknesser it comes out with a bow/cup in it. Anyone any ideas. I'm baffled that the one side I can get flat as a pancake but using the same cutter head in the thicknesser role can produce a non flat surface

 

[Steveinthewoods]

Have you solved this? Is the bow in the centre of the material?