If you are running on Windows, please read this: Create-React-App has some issues detecting when files get changed on Windows based machines.  To fix this, please do the following:

1. In the root project directory, create a file called .env

2. Add the following text to the file and save it: CHOKIDAR_USEPOLLING=true

3. That's all!

For more on why this is required, you can check out: https://facebook.github.io/create-react-app/docs/troubleshooting#npm-start-doesn-t-detect-changes