CSS issue? / Breakpoints not present

i’ve got this plugin working beautifully at one dev site right now and for some reason despite using the same settings at another i’m getting a very different and very broken result, as follows:

1. the responsive menu contents, unstyled, sitting in the footer regardless of screen width,
2. no hiding of the #nav-container div despite the instruction under the advanced setting tab,
3. no sign of the menu button (three lines)

i’m guessing there’s a conflict with my theme somewhere – the breakpoints don’t seem to be working at all (they work fine for the other responsiveness rules in my CSS). firebug seems to suggest no styling has been applied by the plugin files to any of the divs associated with the plugin, including:
#responsive-menu
#responsive-menu-title
#rm-additional-content
#menu-main-menu-container
etc.

here’s the site in question: http://wpd.matmartin.co.uk/
and here’s another site built the same way with the plugin working just fine – the plugin settings are (unless i’ve missed something) the same: http://wpa.matmartin.co.uk/

please help – i’m sure this is some kind of user error but i’ve been looking for it for a whole day now.

thanks.

https://wordpress.org/plugins/responsive-menu/