NFS screwed up

[dirk calvyn puk ac za]

I've just upgraded a machine from RH5.2 to 6.0 and installed
RH6.0 from scratch on another which previously ran 5.2 (yes,
that is the one which I lost the RPM database.)  Previously
they used each other's disks via NFS, but now I get 

mount: RPC: Program not registered

on both ends.  I'm also NFS-ing to two other machines still
running RH5.0, and in these cases have no trouble in either
direction to either of the 6.0 machines.  The only link that
fails is between the two 6.0 machines.

What can I try?

Dirk