What happens when you have dozens and dozens of JavaScript and CSS includes, some hosting internally and some hosted externally? Well, sometimes it is just too much for Google's testing tools to handle and they just give up waiting on a response from the includes.
A recent case posted in the Google Webmaster Help forums shows one site that has 86 JavaScript includes and 21 CSS file includes and Google just is being impatient with it and returning errors in their testing tools, but in the Fetch tool and the mobile friendly testing tool.
Here is a screen shot of the mobile testing tool:
Google's John Mueller replied to the webmaster explaining this saying:
The main problem comes down to there just being too many JS (86) & CSS (21) files used in that page. We have to fetch all of them in order to render the page, and since they're all on the same host (even if one is called "cdn" -- it's the same IP address), we're limited by the time we have available. With that, we end up dropping some of them, and can't properly render the page (it doesn't allow the code to completely run). You also see that with the Page Speed Insights tool, or with webpagetest.org (sample run).The other problem here is that your page has a "meta fragment" on it, but the "escaped-fragment" version of the URL doesn't properly display either.
Ideally, I'd recommend fixing the issue with the number of JS & CSS files regardless of any rendering. This will significantly improve how your page displays, to users & search engines. Past that, you need to decide whether to use the AJAX crawling scheme or not. (We don't recommend it anymore, but we currently still support it.) If you do want to use it, make sure that the "escaped-fragment" version of the URL returns a fully rendered view that properly matches what a user would see.
So keep it simple with all the includes.
Forum discussion at Google Webmaster Help.
