I have created a responsive form and when I open it on my PC it works fine. If I shrink my PC's browser, the form reacts to display the fields in one column as expected. When I open the same form on my Android phone (Chrome or the Samsung "Internet" browser), the full-size form is displayed instead of the single column.
Does it only act responsively in the Nintex Mobile app and not in a mobile browser?
No, it should act the same both in Nintex Mobile App and via the regular, mobile browser.
Maybe there is something with your phones configuration, that it is forcing pages to always display in a desktop mode (ex. How to Permanently Browse in Desktop Mode on Android « Nexus :: Gadget Hacks )? What happens when you open ex. this forum in your mobile browser? Or Google homepage?
Both Google and this site open in a mobile view with collapsed menus, etc. When you open a responsive form on your browser, you are seeing it in a mobile view?
I've attached what I see. The O365 site and list are both in a mobile view.
Can you please do following?