I see that most of your featured images in the slider posts either have portrait aspect ratio (taller than wide), or very small (the trophy image).
The portrait images are quite difficult to fit into the slider. The automatic cropping is evidently inadequate in this case. Try to manually crop the featured images for the slider posts using the Post Thumbnail Editor
plugin. Instructions here: https://pagely.com/blog/2015/06/beautiful-featured-images-crop-thumbnails-wordpress/
For the very small image, I’m afraid you’ll need to switch that with a bigger image. Here are some free images you can use: https://500px.com/search?q=trophy+cup&type=photos&sort=relevance