Why are my previews not working well?

See How do I get my preview images to be more accurate? for making sure the system gets the correct preview image. If it has the correct one but it's blurred or too small, this means that your blog host is downgrading the images for your feed. Take a look at your feed to see if this is the case. You may of may not be able to change this one. But there isn't anything I can do either.