I would say that, assuming you’re using the sample child theme’s functions.php that came with the theme with the custom.css file still included properly as it was by default, if you’re saving CSS on custom.css and nothing is happening on your site, it would have to do with your caching plugin.
So, I’d start there. Try disabling it, and see if you see your custom.css styles being included. If so, then that means you need to get on top of using that caching plugin, whether that’s clearing it after you’ve made changes, or whatever. Generally with caching plugins like that you usually want to wait to minimize and cache everything until after you’ve finished your site.