If using 
wp-minify you need to give the cache folder with the 
wp minify plugin chmod right 777. The same with the plugin config.php file. If you go to the settings of 
wp minify you will see an error message if your writing permissions need to be adjusted.
I am using 
wp-minify and it works great.
Are you using any other plugin that might cause a conflict (another cache plugin for example)?