They won't show because they're 594px wide and they need to be 600px wide (or more).
This is what Staff have said:
Hello all. First of all, apologies for the issues you are having. We have indeed made a change to the way cropped featured images work on WordPress.com.
I'm not even going to question why is has been working this entire time at the image size I had set and now it doesn'
Actually, that has been a long time standing bug with the way we processed images. This has been fixed now. Previously, images that were smaller than what a theme specifies as the size for a featured image would be upscaled and distorted. This should never happen by default.
The side effect is that some of you may have grown used to this behavior. For that, I apologize.
Images that are equal, or bigger than what a theme requires, should still work exactly as before, cropped when needed.
We are checking your reports for possible misses, but, please, if you have a case where a featured image that is bigger than the dimensions a theme specifies is still showing problems, let us know. Thanks. http://en.forums.wordpress.com/topic/problem-with-homepage-2?replies=20#post-724028