The other day, Google launched the mobile testing tool and since then, Google has found two common repetitive trends when webmasters are claiming the tool does not work right.

Google’s John Mueller wrote on Google+ that typically when the tool gives back an error saying your page is not mobile friendly, and you know it is indeed mobile friendly, is when you are (a) blocking GoogleBot from specific resources or (b) cloaking GoogleBot. In both cases, even if your site is mobile friendly, since GoogleBot doesn’t know, it will mark it as not mobile friendly.

John wrote more detail on Google+ to explain:

This is Google once again, asking you nicely, to stop blocking GoogleBot from crawling your CSS and JavaScript files.

Forum discussion at Google+.