Thanks

unconfirmed - I have css compression enabled on my site and the stylesheet is showing as expected.

ensure that the css file was physically written to your files/css folder

We have same problem with the default theme for the drigg module, no solution yet because we don't know the source of the problem, definitively not a file permission problem:
http://drupal.org/node/332118

What do your web server logs say? PHP error logs?
Might be an issue w/ allotted memory for PHP or an issue with Suhosin ( http://www.hardened-php.net ) .

I doubt that this is an issue caused by the theme.

Someone got to the bottom of the problem, see in the thread I referenced earlier. It's about adding lines to yourtheme.info file, so that the optimize css know what to include. It wasn't required in D5, that's why.

I have similar problem. I enable optimize css and js. Some style get lost for the pages sometimes. I check the log. It shows that some css file and js file are missing. The filename is an random filename after turned on optimize option. Drupal delete these files after sometime and the pages can not find these files when next time they load.

I had this exact same problem. What fixed it for me was switching PHP safe mode off in your php.ini file:
safe_mode = Off

the problem arises on non-core themes only.
take a look at http://drupal.org/node/752592

i had the same problem. I'm not sure if this is a help to anyone, but upgrading all my modules and drupal to the latest versions fixed the issue for me. It didn't have anything to do with the theme.

We also have the same problem with our website. If we disable the optimize css the theme just crashes.
The css files are even deleted after a while. The very strange thing is that we haven't edited anything on the website for a few weeks or so (we've been working on a development site).
I did all the possible module updates but that didn't do anything. We're still looking into it.
And we are using Drupal 6.19.
If anybody has an answer it would be very welcome.

I just update my drupal core 6.13 to 6.19
I see now the code of the batch on the common.inc file.

But now my theme crash when enable the optimize css files.

Does someone have the solution ?

I found the solution.
You just have to convert all your css files in unix/utf8 and it's done.

I was having the same problem with another theme and your solution solved my problem :)

I was saving the css files I created in ANSI, but converting them to UTF-8 made them worl with Drupal's CSS optimization. Thanks!