Hi,
the problem is that RGT VIs use classes so if you add a new VI to the RGT the VI MUST be part of the class, otherwise the VI does not have access to the class and can not be executed. Please have a look at those ressources about Object-Oriented Programming:
Introduction to LabVIEW Object-Oriented Programming - National Instruments
http://www.ni.com/webcast/2703/en/
LabVIEW Object-Oriented Programming FAQ - National Instruments
http://www.ni.com/white-paper/3573/en/
Best regards,
Christoph