Rarely -I cannot connect to shared resources from certain computer

[ABC]

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".