What export settings are you using? I suspect the settings you chose flatten transparency and you need to check the Simulate Overprint box.
@Peter – Flattening transparency should be no reason for this.
I tried something like this and exported to PDF/X-1a. See the following screenshots:
1. InDesign: Group selected, knockout group option selected:
2. InDesign: Fill and FX settings of the top most circle in the knockout group:
3. Inspected in Acrobat Pro with "Simulate Overprint" is OFF and Show Overprinting is ON (all overprinting objects would be shown in a special color = light blue):
We need more information from Kjell and maybe some screenshots to solve this riddle…
Exporting to PDF/X-1a must flatten the transparency before exporting, by definition.
You will need overprint preview on to correctly view all exported PDFs. Recent versions of Acrobat will turn this on unconditionally if they see a PDF/X file.
As you can see in my posted screenshots some simple examples like mine do not need overprinting at all after flattening. Fragmentation (stitching) of objects does the magic in this simple case:
Since we do know next to nothing how the OP constructed his knockout group we cannot give any advice.
We also need to know the used export preset.
It seems my problem went away when I used the export profile my printer gave me, which involves a custom flattener setting.
Thank you anyway for your help, I'm sorry if I didn't provide you with sufficient info on my problem. Like I said, I'm kinda new to InDesign.