Thanks for your quick answer!
It's happening in the Go overprint. I print on 1440, unidirectional.
It's not always happening but I noticed something: when I select my Go image(small 4cm square 255,255,255 image) and want to print on an A4 paper size(210x297mm), it mostly looks like a small square white in the preview.
When I see this small image in the A4 preview, the Go will printed ok.
But, sometimes, the Go file looks much bigger in the preview and when I print then, the problem occures.
It seems in the print that the Go print missed some dots on a very regular base, I can try to make a decent picture or scan later and upload it.