style.css doesn't always work…

I’m running into an issue where creating new div’s id’s and classes and styling them via my style.css doesn’t always yield results. Im wondering if I set up my child theme incorrectly. This is my first new install, so I’m learning new things every day. Sometimes I learn that what I did a week or 2 ago wasn’t the “right” way. Here’s my situation:

I downloaded and installed a theme. I duplicated that theme into a child folder on the same level as the theme folder, I renamed my style.css sheet and I gave the child folder an appropriate name. This worked, so I went on to build a nice little website. As I have learned more about html and css, I have been adding custom code via the wordpress text editor on specific pages. Sometimes the css does what it should, and sometimes there is no change. I check my html and css with jsfiddle.net, and it works. So then I do inline styling in the html, and it works. So I’m wondering If I didn’t set up everything just right when I made my child theme.

For you experienced developers, I’m sure there are little alarm bells going off in your head already and I’d like you to share them with me! Its time to start making my content responsive and I need for css to work for me! Thanks!