set_post_thumbnail_size not cropping featured images

I may be misunderstanding you, but it seems that it is because your image is not 250px high in the first place. What is the original image dimension?

It’s not really described well in the Function Reference, but 460 will the maximum width that an image can be, and 250 will be the maximum height. crop = true simply means that if either of those dimensions are bigger, the image will be cropped to fit, as opposed to resized.

For example, take an image that is 690px by 500px –

/**
 * This example will crop the image to just 460x250, therefore cutting off
 * the excess pixels
 */
set_post_thumbnail_size(460, 250, true);

/**
 * This example will resize the image to 345x250, so that it fits your desired
 * dimensions while at the same time keeping the whole image
 */
set_post_thumbnail_size(460, 250);