Hi,
Another possible reason could be that clients might be getting authenticated from different site domain controller instead of local site. Please provide us
1)ipconfig /all - domain controller from site 1
2) ipconfig /all -- domain controller from site 2
3)ipconfig /all -- from client PC which is having problem
4)nltest /sc_query:<domain> -- from client PC which is having problem.
Thanks,
Umesh.S.K