Responsiveness on mobile device

Hi there,
I have a problem with the responsiveness of my whole website (under Boldial) on mobile device: the columns are supposed to stack in one vertical column but they don’t, they just skrink on the same row.
As a result the text cannot be read and the images are very very small…
There is a problem with the navigation bar on mobile device as well: the navigation bar sticks to half the mobile phone screen or more !
When I created the website everything was fine, I don’t understand what changed since then.
Is it a problem with the pixel width in “Responsive layout” or in “Header navigation responsive layout”? I tried to change them with no better result.
Thanks in advance !
Marie
http://www.macassar.fr