Aggregate and Compress CSS Files

Hi

Everytime I select 'Aggregate and Compress CSS Files' in D7 Performance settings on my live site I'm instantly losing my styles. However on my local machine this does not happen. Interestingly this only happens on my live site when I have my sub-theme of Bartik selected. If I use the default Bartik theme there is no issue. I've searched Google and found a plethora of possibilities but I still haven't found the solution. If anyone could advise it would greatly appreciated.

Thank you

Stephen