I just wanted to share this tip that helped me fix my problem. Maybe it will help other users.
I normally use the Brave browser. When I enabled all 5 of the recommended CSS and JS optimizations, they appeared to be working OK when I viewed several sample web pages, while I was logged in as a WordPress admin.
But, several days later, I accidentally noticed when not logged in to WordPress, or when using a different browser (Firefox), that images were not loading and drop-down menus were not appearing on hover.
By turning off optimization switches one by one, I traced the problem to Combine JavaScript Files. I looked at the Exclude from JavaScript Combination drop-down list choices and excluded two JavaScripts for my theme (MTS Schema) [ /themes/mts_schema/js/customscript.js and /themes/mts_schema/js/layzr.min.js (for lazy load images)].
That solved both problems, and I verified it by checking the sample pages on several different browsers.
The optimizer and cache have given a big boost to my page loading speed. Thanks for the valuable plugin.