I have been having the same issue with a trade show booth I was designing. I was working at 50%, but the placed image had to be 300 dpi. So you can imagine it was still a very large image. I was placing the image, but all I could see was a blank box. I tried just about everything suggested here and then some. What worked for me, in the end, was reducing the file size by cropping it as much as I possibly could in Photoshop (previously, I was using a clipping mask in Illustrator). Apparently, Illustrator has a max file size it will accept for a placed image. I have seen no documentation on this. Just going by my experience. I hope this info is helpful to someone.
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