This can be caused by one of two things-
1. The image was printed as a composite rather than separations
Even if the image is a single color, separations should be selected. Under File > Print, select the the Color tab and make sure you have separations selected.
2. Convert Spot Colors to CMYK is checked
Go to File > Print and under the color tab, make sure this box in UNCHECKED.
See the attached PDF for a visual.