activating certain plugins break the child theme from loading properly

I'm having a weird issue with a child theme and some plugins - the theme is from OBox and is a child of Loft. The site is quite complex, with lots of features from plugins.

The problem I'm getting is when testing in internet Explorer 9 and lower - if I turn on ANY of the these plugins (PPM Accordion Ultimate / Responsive Styled Google Maps / Simple Tooltips), the styles form the child style.css file don't seem to load at all, breaking the site.

I have a feeling this is something to do with the either the way the stylesheets are encoded or that they're using @import to pull in additional stylesheets - I've read somewhere this is not supported by IE 9 or lower.

Am pulling my hair out, have tried so many things I can't remember what does what.

ANY advice on how to fix the problem would be a HUGE help!
Aaron