This is a 2 year old thread. It’s likely that whatever issues there once were have long since been solved. And any bugs in RapidWeaver have long since been repaired.
(side note: I am pleased to see that site is still using RapidWeaver and Stacks even after two years and a rough beginning. )
Probably best to post a new question with specific details about the problem you’re facing with screenshots and info about what stacks/themes you’re seeing the problem in.
It generally comes down to the theme and the stacks that you are using. Most of the Stacks 3 elements (as well as 3rd party ones) are responsive by default. e.g. – the 3-column stack will automatically rearrange to 1-column on mobile devices.
But the theme is important too. Make sure that you are using a theme with a flexible width or that is responsive on its own. If the theme forces a specific non-responsive width then all the content (including the stacks inside it) will oblige and be non-responsive too.