Images won't display ...

I have loaded 1563 images into a project. The project shows that there are 1563 images in it, but it will only display 1062 of the images. For some reason these 501 (consecutive) images, although apparently in the project, won't display. I have tried quitting out of Aperture and even shutting down the computer and restarting. No change. The browser is set to display unrated or better so the ratings shouldn't be having any effect, none of the images have been rated anyway. The images were created using a canon 30D and were shot in Canon RAW. Any idea what could be happening? This is very frustrating.

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