N Computing Network Connect Problems

I'm administering a computer lab running N computing technology. At time, all of my virtual machines connect to the server and all users using those virtual machines are able to connect and use the machines. At another time I have trouble connecting those virtual machines to the server. What happen is this: The virtual machine can see the server, but one a user wants to connect using the virtual machine, he/she cannot connect yet the server name is being displayed on the virtual machine's screen.
I"m having headache troubleshooting this, and I would like to ask anyone who has got deep knowledge and experience working with N Computing Technology to assist me. This post is opened to everyone who has networks knowledge.