Google recently has sent a warning email "Googlebot cannot access CSS and JS files" to webmasters. In the email, Google said that the ranking will be affected if it can not access your CSS and JS files (quote: "Google understand that your website works properly so blocking access to these assets can result in suboptimal rankings"). This may be came from changes in Google's algorithms recently especially on mobile.
The warning email also shows you a solution to fix, but it may cause you take time to follow or research. Actually, you can solve the problem simply by modifying the robots.txt file on your website (e.g. http://yourwebsite.com/robots.txt). Just allow folders that contains your CSS and JS files. Below is the robots.txt file of my Joomla website after modified:
The syntax to allow is: Allow: /thefoldername/
You can use "robots.txt Tester" tool in your webmaster tool to verify again to make sure Googlebot can access these files well. That's all.