What type of client host is it? Is it a LPAR? Standalone machine? P4, P5?
Is the client connected to an HMC? If connected to a HMC (or it has a console display), have you booted the host into SMS and then changed both the boot device order and the remote ipl options to tell it how to connect to the NIM server?
If so, do a ping test within SMS to see if the client can communicate with the NIM server and let us know the results.