Pages showing without CSS in preview

This has just started to happen, but not on every page.

I create a new page (copy existing page in same folder and the edit with new information etc)  and when finished I check that it looks and works by using the Open in Browser command. Some pages as I have mentioned open fully but some look as if the CSS has been left out or is pointing to the wrong folder. 

I have checked that pages have the correct CSS which they do. I have attached some examples of the issue. 

When I check other pages that link the pages that don’t open directly from the Open in Browser do work as expected.

Any help would be greatly appreciated.