Hi, Michele. From your question, I think you are saying that some users are being authenticated successfully via LDAP, and some aren't? If that's the case, make sure that all users are in a container that can be found under your Active Directory root DN. We had seen an issue like this before in our own environment, and had to ask the network administrator to move some users to the standard container in AD.
If no one is being authenticated via LDAP, you may have an issue with the LdapRootDN you have specified. In our (working) environment we have it formatted as OU=[AD root container],DC=[subdomain],DC=[domain],DC=[tld], e.g. OU=users,DC=ldap,DC=mydomain,DC=org
I hope that helps,
Jamie