Rarely -I cannot connect to shared resources from certain computer

Hi:
I’ve a problem and I need help with this issue. The situation is described
as following:
I’ve 2 sites, “Site A� and “Site B�. The computers in “Site A� cannot access
to the most shared resources of computers in “Site B�. However from the
computers in “Site B� can access to their shared resources between them as
well as to the computers in “Site A�
The computers in “Site A� can do ping and get remote access by terminal
server to the computers in “Site B� and vice versa. However from “Site A� it
cannot access to the shared resources of the most of computers in “Site B�
and for other computers in “Site B� it can be access it.
The client computers are managed by active directory domain, i've 2 Domain
Controllers under Windows Server 2008 in "Site A" and 1 Domain Controller in
"Site B". The clients are Windows XP. The PDC is in "Site A".