File looks black from Preflight to imposition, yet rips with a screen??

We are new in the CTP world, but have a problem thats driving us a bit crazy. We preflight our PDFs (in Acrobat), making sure output will be a single color or spot color, file looks fine. Place the same PDF in Indesign in position, still looks fine. Send it to the rip, and every once in a while a part of it will show up screened. Typically this is happening with bold words and logos. Anyone have any idea what is going on?

 

What colorspace is the InDesign file using?

If the InDesign file is using an RGB colorspace, instead of CMYK, when you print or save a PDF from it, the print engine is taking the RGB info and converting it to its colorspace. That can leave screens, even in solid black.

That would hold for placed logos, etc. I used to open Illustrator files, open the CMYK color sliders and make objects solid K. Then I'd save and print or place them in an InDesign document and get screens. That is, till I realized the color space in the Illustrator file was RGB and had to be changed to CMYK, (even in an all Black file).