Assume, despite warnings about the potential problems allowing Moodle UI to create users in AD/LDAP, that you are using M$ AD/LDAP. If you have the LDAP info correctly entered into Moodle, can another user whose account authenticates via LDAP login?
Got command line? There is a command line script that can sync LDAP users into Moodle. moodlecodedir/auth/ldap/cli/ and it's called sync_users.php. No parameters need be given the script as it reads the LDAP information from Moodle config.
You are going to have to check both ends ... Moodle error logs AND the logs of your LDAP server. If you don't admin the LDAP server contact server admin. Could be the bind user setup in Moodle doesn't have permissions to create new accounts.
'spirit of sharing', Ken