This is usually due to the dimensions of the uploaded image.
We restrict the size of printed products people can order to those where the dpi of the resulting image would be a minimum of 150dpi. Anything with a dpi less than this would result in a pixelated image that we don't feel would be of a high enough quality to show off a photographers images to the best extent.
If certain print sizes are greyed out, this means that the uploaded image is too small to meet this 150dpi requirement
We recommend photographers upload files with as large a dimensions as possible, images with a height/width of 3-4000 pixels and above will greatly increase the range of print sizes at which images can be offered.