Have you already setup and ran the Regenerate Thumbnails plugin?
Have you made sure you’re settings are 1140px in your Slider Settings page?
If that’s not it, right click your image while viewing from the slider, and look at the image name (should show up at the top of the browser when you open it)….what does it say? Does it say “cropped” or “960” or anything smaller than 1140px? If so, then that means WordPress cropped your image previously and is using the smaller version instead of the large/original size.
If you have a link I can take a look as well.