Clear the browser cache: Sometimes, the browser cache can prevent changes from being immediately visible. Clearing the cache can help to ensure that the latest version of the file is being loaded. or
Check the file paths: Make sure that the file paths are correct and that the files are being served from the expected location.
I have also noticed that when you upgrade your Mendix application from 8 to 9 it will still need Calypso for compiling your CSS files into SCSS.
This will probably come from the changed file structure in version 9.xx
If this is your problem you should check this documentation out to fix it: https://docs.mendix.com/refguide/moving-from-8-to-9/