i can't mount nfs share as rw

[Tomoki Taniguchi]

I am trying to export a folder from one server using nfs
and mount it on two different clients as a read/write enabled
nfs share.

i have been following the instructions on (except for locking down
portmap for now)
https://help.ubuntu.com/community/SettingUpNFSHowTo

1) apt-get install portmap nfs-kernel-server on the server
2) apt-get install portmap nfs-common on the clients

3)setup the export lines on /etc/exports
/nfs-exports/xen-domUs          x.x.x.1(rw,sync) x.x.x.2(rw,sync)
4) run "sudo exportfs -a", restarted portmap and nfs-kernel server

5) on the client i mount the exported folder
mount -o rw x.x.x.3:/nfs-exports/xen-domUs /xen/domUs

6) i confim that the nfs share is mounted rw by listing "mount"
7) i go to /xen/domUs and confirm the contents of the shared nfs exists.

but everytime i try to alter anything in the nfs share on the client,
i just simply get
rm: cannot remove `this': Permission denied
i can perform the same exact thing when i am directly on the nfs server.

what am i doing wrong here?  why can't i write to the nfs share?
both clients and the nfs server is running feisty.

TIA,
Tomoki