The mobile testing tool analyzes URLs to determine if pages have a "mobile-friendly" design. It is a useful resource as it shows if those websites are using software (like Flash) that isn't compatible with mobile devices, uses text that is unreadable (without zooming), or if it sizes content appropriately so users don't have to scroll horizontally. While many website owners think their website is mobile-friendly, Google's John Mueller indicated there are two issues that may prevent the tool from working properly and prevent your site from garnering the now highly coveted 'mobile-friendly' designation on the search results.
Since the tool was released, Mueller revealed, Google has noticed that some websites are blocking files (through robots.txt) such as CSS or JS files that are necessary for a mobile version of a page to load (and for the mobile testing tool to work).
The lesson? Stop blocking (and cloaking for) the Googlebot.