Some black images are faded in PDF

I have an image that was created in Illustrator CS3 with a black background (C=0, M=0, Y=0, K=100). I converted the .ai to .pdf (Acrobat DC) and imported it into Framemaker 12. It looks fine there, but when I print the Framemaker file to .pdf, the black in the image is a lighter shade than other black objects in the document (this is made obvious because I put the image on top of a larger black box, which has the same CMYK values). I tried converting the .ai to .eps instead but in the framemaker-to-.pdf output, the black was still faded. How can I fix this?