Duplo DC-615 skewed creases

Hello,
The creases I produce with my Duplo DC-615 are always slightly skewed whatever the settings (crease depth / skew adjustment). The skew adjustment knob seems to have no effect on crease skew while it correct very well cuts skew. Any idea what to look for ?
Thanks.

 

With the run direction or across?
Are your rollers dusty or clean?

 

I apologise for replying so late. I don't remember asking this question here, but the problem remains. Thanks for your reply. The rollers are clean. However, I don't understand the question about the direction: on the 615 there is only 1 creasing tool which is perpendicular to the transport direction. Whatever format I run through the machine and whatever the skew compensation setting (at the input), the creasing is always slightly skewed.