You may notice that some of your grayscale images appear as blank gray thumbnails in the Import window, while others don't.
This issue affects 16-bit grayscale TIFF images. However, other grayscale images, such as a JPEG, may work as expected.
Aperture does not support 16-bit grayscale TIFF images at this time. As a workaround, try using another application to convert the grayscale image to RGB mode before importing to Aperture.