A number of things can go wrong with EPS images. The most common are:
1. That they are saved in a state that does not allow editing. FreeHand was reviled for this "feature" in the past, but has since abandoned the practice.
2. They were created on a Mac, and transferred to another platform without being compressed in StuffIt or Zip format. This can lead to problems since the actual image data is stored in the resource fork, not the data fork by some applications. This is especially true of applications in Mac OS 9 or earlier. Transferring the images via mail or ftp often lead to a loss of all resource forks if the correct settings aren't used in the process.
3. That they were created on one platform (Mac or PC) and someone tries to use them on another (PC or Mac). Preview images don't show up properly on the other platform unless you take special precautions when you save the images.
Try mounting the images in your layout application, and even if they don't show properly, try printing the page and see what happens. A lot of times EPS images will print, even when they don't show properly in the layout. This is especially true for images created on one platform (Mac/PC) and used on another (PC/Mac).
If they don't mount, or don't print, they are probably corrupt in some way. Try getting the person/persons who created them to supply you with new copies. The best way is to compress them using Zip or StuffIt formats, before putting them on the FTP server.