Negative score in Performance/Image Optimization

When I run a test in /My websites/Performance I get different score everytime. I smushed all images on my website and I am still seeing score for optimizing images 11/100. Its changing everytime I run new test even though I didnt make any changes on my website but its always negative. How can I improve Optimize images score?
It looks like WP Smush Pro didnt make any performance optimization for images displaying on my website.