ENDIAN Firewall - IPSec and Doamins

Hi there,

I have 2 endian firewalls configured... one for site A and other for site B - they are integrated via IPSec and I can ping any machine from/to both sites.
Besides that I can still make Remote Desktop connections, connect to database from/to both sites but CANNOT JOIN A DOMAIN from site B to site A.
Site A has a domain controller and PCs (all added to the domain), and site B has only PCs (need to add them to domain in site A).
I tried 2 types of domain controllers: MS Active Directory and Samba, but the problem is always the same: "The specified network name is no longer available"

Can somebody help me find out what is missing?

Thanks!