CSS for slide does not work when I place it in external css

CSS for slide does not work when I place it in external css

Hi there, am working on a slider for a client site and all works fine when I place the css directly in the page - but when I move it to external file it does not pick up any of the styles that are required to make the slider work.

An obvious mistake - most probably so apologies if being a bit dumb - any help greatly appreciated.

Thanks in advance