NFS server on two networks

I've set up a Linux NFS server with 2 network cards on 2 different
networks.

Network A) eth0 has IP 151.89.63.10 on the net
151.89.63.0/255.255.255.0
Network B) eth1 has IP 172.31.1.16 on the net 172.31.1.0/255.255.255.0

NFS works fine on workstation on network A, doesn't work on network B.
Networks are identical and NFS clients (workstations Linux) are, too.
Everything is open, I can ping, telnet... almost everything.

This is the error on the workstation on network B when trying to
mount:
# mount gess10:/nas3 /CS1
mount: RPC: Port mapper failure - RPC: Unable to receive

This is the "/etc/exports" file:
/nas1 151.89.0.0/255.255.0.0(rw,insecure)
/nas3 172.31.1.0/255.255.255.0(rw,insecure)

(I tried also with 172.31.0.0/255.255.0.0).

NFS Server is Red Hat 9.0
NFS Clients are Red Hat 7.3 (but on A network it works also on Digital
UNIX)

My questions could be: does NFS answer on the A card only? Should I
configure nfs/rpc to manage double network?

Thank you in advance,
Massimo