In dev you have to make sure that in your security.yml
the security checks are disabled for the resource routings:
firewalls:
dev:
pattern: ^/(_(profiler|wdt)|css|images|js)/
security: false
In prod environment you need to dump the assets with the command:
./app/console assetic:dump —env=prod —no-debug
See also: https://groups.google.com/forum/?fromgroups#!topic/symfony2/u49SHQaxZS0
See further: How to integrate the lessphp filter: https://gist.github.com/3049271