Preview of .eps files don't always appear

Can someone shed some light on this weird situation. I have a large collection of stock photos and art. Sometimes as I look through my folders, the preview image of .eps files does not show in coverflow or other window views. This is such a pain, because I actually have to open the file up in Adobe Illustrator CS5 to see what the image is. The really wierd thing is that sometimes the previews are there. But most times, just the standard ".eps blank document" icon shows. Any suggestions?