There are a couple of things that could be going on, and as an aside, trying to match a solid background to a sampled area in an image is seldom successfule as there is too much variability in the actual color in most images.
The first possible issue is that the sample you are taking could be inaccurate. Are you sampling in ID or in Photoshop? You mentioned CMYK, and I don't believe ID will sample the CMYK in a placed image, but uses the preview instead. If you are sampling in Photoshop you are either sampling an average of the pixels in a square area, or a single pixel, depending on the eyedropper settings, and either choice is likely not to be truly representative in many cases. A further complication is if you are sampling an image that has a differnt color profile from the working space in ID.
The other big possibility is that your printer uses what is know as a dual-layer or CT-Line (for continuous Tone/Line art) processor and handles the raster image differently from the vector object created in ID. Theres a pretty good explanation of this in InDesignSecrets » Blog Archive » Eliminating YDB (Yucky Discolored Box) Syndrome
Hi, I am sampling from within ID using the eye dropper tool. I will try each of your recommendations and see which works. Thank you for your help