Oracle Database Discussions

I have an app that connects to remote databases and creates data table from desired table for viewing.
My parameter dialog pops up and the following is entered:
Host Name, Service Name, Port, User Name, Password.
The "Test Connection" button passes these into the method for this string:
var connString = string.Format(CultureInfo.InvariantCulture, "Data Source=(DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST={0})(PORT={1})))(CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME={2})));User Id={3};Password={4};", HostName, Port, ServiceName, UserName, Password);

I try/catch for various OracleExceptions, and if none, the params are passed into the worker so real data can be pulled.

I have several databases where this works fine, but when testing a new remote database, I got the ORA-12547 exception.
I have seen this question several times when someone has changed something, but I haven't changed anything.
What does this exception mean in English?
What do I need to be looking at to find where my problem lies? (it doesn't seem to be my code because some connections work!)

tks