Can you make the below verification to see whether your encryption and decryption works correctly or not..
In Old Server
OPEN SYMMETRIC KEY [key_name] DECRYPTION BY CERTIFICATE key_cert;
GO
SELECT encryptbykey(key_guid('key_name'), 'TestData')
GO
In New Server
OPEN SYMMETRIC KEY [key_name] DECRYPTION BY CERTIFICATE key_cert;
GO
DECLARE @blob varbinary(8000);
SET @blob = 'output from earlier select statement'
SELECT CONVERT(varchar(8000), decryptbykey(@blob));
GO
Close symmetric keys in both servers
CLOSE SYMMETRIC KEY [key_name];
GO