I was able to log into @Spud1's site, and found the issue was due to incorrect permissions on the styles folder. Details below.
The short answer:
If CSS is being cached to disk,
wp-content/uploads/styles (the folder) and cache.css inside of it should have permissions set to 755 or 666.
The long answer:
This particular site had the
wp-content/uploads/styles directory permissions set to 766. This causes files within that folder to be WORLD writable, as well as executable. This is a big security concern, especially on shared hosts. It can cause a site to get hacked, fast!
That particular site was hosted on Host Gator, which has a very good security policy for sites on shared servers: Host Gator flat out refuses to serve files that are executable and world-writable. This is a great policy, because it protects users on those sites from a very common attack vector caused by often difficult to understand permissions settings.
Changing the permissions back to 755 caused Host Gator to serve the files again without complaint, resolving the issue.
No changes were need to the plugin. This problem would have been the same for any file on any site, whether it was using WordPress, Styles, or anything else.