Q: Images diappear after I move a project to a folder

When I'm dragging projects into folders, Aperture (3.2.3) typically at some point ceases to display the images in the projects in their new locations. Once this happens, it continues until I quit and relaunch Aperture. I know by now that the images are there, not lost, but it's annoying that I've found no way to get them to display except to relaunch Aperture.

Aperture knows that the photos are there: in the Library Inspector, the number of images in the project is shown correctly. It's not the location of the project: if I undo the move or drag it elsewhere, the images still do not display.

Does anyone else have this problem? Is there a way to prevent it? Failing that, is there a way to get the images (thumbnails, I guess) to display again without quitting and relaunching Aperture?

Thanks for any help.