LDAP returns "Credentials Invalid" error from service but not from IIS Web App

I have a windows service that runs under the same credential as the app pool for our web app. When I logon to the web app the LDAP authentication works fine. However, when I call an API implemented in the windows service passing in the same credentials, LDAP server returns "Invalid Credentials" error

Windows Service uses the same DLLs that call out to the same LDAP server.

Am I missing any configuration/permissions on my windows service? The service is currently hosted on the same box as ISS.

Thanks In Advance