Advanced CSS/JS Aggregation Module Breaks My Site Layout
I faced the same issue, here is some steps to check:


check if your css loaded and optimised in your page source code some files like /sites/default/files/css/css_QnKw33sohmgUyVqs9lGXvHTJTZsphXTkXdkVVmt25ts.css if it's no check if there is any errors in your console, if this is the case Check your status page and log messages, and permissions
If it's yes all files well loaded this is was my case i had some css errors that's why even my css files loaded not taking any effect:
So i had to check / validate all my css files and correct any errors.
I used this https://codebeautify.org/cssvalidate.
there is a lot other websites to validate your css.
I hope this help you !