layout-fixed.css not displaying on some IE pages

I am having a problem with how some of the pages of a site I am working on are displayed in IE:

If I am logged in, the public-facing pages of the site display correctly (screenshot1), but on the administration pages, all the styling controlled by layout-fixed.css seems to have disappeared (screenshot2).

When I am not logged in, the layout styling doesn't display at all (screenshot3), which is odd as the pages this applies to are the same public-facing pages that display fine when I am logged in.

From looking at the source code, it seems that this stylesheet definitely is being called on these pages where it doesn't seem to display, so I'm not sure why IE doesn't seem to be picking up on it - particularly as several pages seem to be displaying fine.

There are absolutely no problems in Firefox, Chrome, Opera or Safari.

I have tried optimising the CSS files, but this had the effect of disabling all styles everywhere on the website.

Does anyone have any ideas on what might be causing this and if there is anything I can do to fix it? Any advice would be really appreciated as I'm completely at a loss as to how to resolve this.

Thank you!