I had a similar problem with a site I’m working on this week. What worked for me was not only changing the dimensions of image in ‘add_image_size’, but also I had to delete all the featured images and re-upload them. You can keep the same posts.
If you notice the images that are being used in your slider are not the same as the images you have in your posts. My guess is that the theme crops the images at the upload stage.
