InDesign :: photo's do not knock-out

[Ulrike]

We're searching on a solution for next problem:
When we place photo's in a magazine that cover or overlap another element (like a black or colored filled frame, or a percent filled frame) it's NOT knokking out the color where the photo should be, but the color is running under the photo when we put out the films. It's not happening when 2 photo's are over each other.
Therefore we use the trick to fill the back of the photo with white, but let's say there should be a better way...

Someone any idea ??

Thanks, folks.
Ulrike