NFS issue a network boot

We are trying with PXE Network boot with entire root file system mounted via NFS. We are booting multiple PXE clients from the PXE server set-up. In some PXE clients, we are getting error messages "nfs: server <ip address> not responding, still trying" and system is stuck. We found other PXE clients are working fine at the same time. Request your comments and suggestions in resolving the issue.