I'm not sure this is the correct answer for you, but:
Sometimes I've found that I'm missing pieces of code from the parent theme that I didn't think I needed.
In other words, are you sure that the code snippet you are pasting is complete? Bootstrap seems to have several snippets in different parent locations that ALL need to be pasted into the child theme. Not sure this is right...just a thought.
Another problem you might come up with is the following:
Depending on the hosting service that you use, AND whether or not you are using a CDN (Content Delivery Network), your may have a caching service enabled that delays any changes to your Child Theme. Sometimes just deleting your cache, logging out, and then logging back in will then display your CSS changes.
Every time I make changes to my CSS, I install WP Super Cache (choose your favorite plugin), enable it, delete my cache, and then uninstall the caching plugin. It seems to freeze up my site when I leave it installed. You might be able to delete your cache from the server side without installing a plugin.
Maybe one of these ideas will help you out.