delay on change user permission


I administer an mac osx server 10.6 at an italian college.
I have the following issue:
When I add or delete a group to a user (using Workgroup Manager or dscl command line, it's the same), the effective permission application delayed for more time (one hour or more...)
In the directory the record are ok, but if print the id user from command line, the added group don't appears...
After a long time it's appear and the permission are effective.
There some daemons that synchronizes the record?
When I create a user the application of permission are immediately instead.
This it's very annoying!
Thank you.