Account Disable

I have Termainal server with sp6. I have set the account policy to the following:
Enforce password history = 3 passwords remembered.
Maximum password age = 30 days.
Minimum password age = 1 days.
Minimum password length = 6 characters.
Passwords uniqueness = 12.
Our Account lockout policy settings are:
Account lockout duration = 21600 minutes.
Account lockout threshold = 3 invalid logon attempts.
Forever (until admin unlocks).

I have individual accounts that are set with PASSWORD NEVER EXPIRE. But every 30 days the user accounts are automatically set to account disable. Any suggestion would be helpful.