Trust relationship

I have 3 domain: Domain A (NT4 domain)and Domain B(2000 Domain) in Site1, domain C (NT4 domain) in Site2.
Domain A and B had established trust each other, and same as Domain A and Domain C.
However, it cannot establish the trust between B and C. The error said it cannot contact the other domain.
The connection from Site1 and Site2 is fine, otherwise Domain A and C should not have the trust establised.
There should have no problem creating trust between 2000 domain and NT4 domain, or else Domain A and B should not have the trust established.
From both Domain controller (DC), they can ping each other’s netbios name, domain name; so there should not be any problem with it.
It is very strange. Anyone?