Hi Sophie,
While mobiletest.me works to demonstrate how media queries change layouts, it does not tell ContactUS.com its a mobile. You are looking at different screen resolutions. So, the Tab will be the screen tab on everything related to screens, atleast in Mobiletest.me Try looking at it from your mobile phone. And also go to the forms mobile settings and make it display the mobile optimized tab and form is on. Please check this article out:
http://help.contactus.com/hc/en-us/articles/200676106-Mobile-Settings
If that does not work, let us know, as we have thousands of mobile tabs working correctly. So, I'm sure we can help you get there :)
