SQL Network Interfaces, error: 26 – Error Locating Server/Instance Specified

Hi, Here is the brief outline of my problem.
SQLDC is a SQL server  R2 dommain controller.
SQLA and SQLB are SQL server 2008 R2 members on which I installed SQL server 2012.
SQLA has a default database TESTDbA and an instance SQLA\INSTANCEDbA
SQLB has a default database TESTDbB and an instance SQLB\INSTANCEDbB
From Explorer Object of SQL Server Management Studio of SQLA, I can access to TESTDbA,
SQLA\INSTANCEDbA and TESTDbB but not to SQLB\INSTANCEDbB.
I tried many variations.  Even when - I prefixed the server/instance name with tcp
(tcp:SQLB\INSTANCEDbB): and when I specified TCP/IP 
for the network protocol in the connection properties dialog (tcp:SQLB,49226) - yielded the same error.
What else to do in order to troubleshoot this issue?