by Andranik Sharabchian.
Hi Siamak,
I've already solved the problem. You are right, for LDAP authentication it is advisable to turn off Moodle password policy. But it hadn't been my problem. Security in our organization doesn't allowed computers to connect to another IP in the network. Only a few computers have this permissions. This is the cause why only a few computers can connect to LDAP server in our network and authenticate self in Moodle.
Thanks for your reply.