This will happen (only)… if your theme has been customized wrongly.
For example, many lazy developers hardcode, or directly add their css and js files to the header or footer files. When this is true, there is no way for the plugin to process them, thus you will end up with more css and more js files (the ones you hardcoded + the new ones generated by the plugin), giving you a worse speed score.
Further read: https://wordpress.stackexchange.com/questions/196841/enqueue-styles-through-the-wp-enqueue-style-function-vs-link-tag-inside-header-p
Unfortunately, this kind of bad practice, “give me your money fast” coding is recurrent when hiring cheap freelancers, OR when the site owners do changes on the site they don’t fully understand.
I suggest you to hire a proper programmer / developer, to refactor your modified theme, in order to properly enqueue the styles and javascript files, because things are bound to break in the future, due to some update or whenever you need some new plugin (such as this one).
If your theme is correctly developed, without errors (such as a commercial theme from themeforest, or most free www.ads-software.com themes), this plugin will for sure give you a much better score (usually a 20 to 30 points increase).
Furthermore, this plugin is meant to be used by people that understand the settings, what they do and what they mean. It’s not meant to be a magic “plug and play” plugin form site owners looking for a faster site.
Using this plugin alone, I often get sites score on 90+ (and you can read other reviews also), so the problem you report, is almost certainly not the plugin responsibility, but rather the theme that has been poorly coded.
Also note, that using other plugins with the same objective (such as multiple plugins for speed optimization), will conflict with each other. You should only ever use 1 plugin for the same thing.
Finally, before posting a review, you could havve asked for help / feedback on the support forum: https://www.ads-software.com/support/plugin/fast-velocity-minify/ and I would have tell you, what I said above.
If you’re interested on fixing your site and having a better score, you can see my profile on where to hire me, or you can look at upwork.com for other freelancers to help you fix your theme.
Kind regards,
Raul
