As you see, it's only happen on one of the images (Height of image is too big), that's why you should do what Bryan told you.
And it's not working probably because you provided too big value of height or width for the images in theme options>blog,portfolio&shop>featured image.
If you'll provide too big width or height there than original image has, then it will not gonna change, it will stay with the same resolution.
Please do what Bryan told once more, but this time check, if you didn't cross the original size of any image.
If the value of height and width of the featured images you set in theme options>blog,portfolio&shop>featured image is higher than the actual image you have in your media library, then it will not resize that image -because the image is smaller and cannot be stretched to fit the featured image size.
Comments
Remember to use thumbnail regeneration after you make your changes. Otherwise, the images will not change their size.
thanks