re-export NFS mounted disk to be NFS mounted?

Hello,

I have a cluster, the disk space is on a Xserver, it's NFS exported,  
and mounted on the cluster's head node without problem (mounted on / 
mnt/nfs), then I exported the "/mnt/nfs" directory, then tried to  
mount it on the nodes in the cluster (mount -t nfs headnode:/mnt/nfs / 
mnt/tmp), it gave error: mount ... failed, reason given by server:  
Permission denied.

my Xserver nfs exports entry has this: /Volumes/DataRAID  -alldirs - 
maproot=nobody -sec=sys -network my.headnode.network -mask 255.255.255.0

my headnode mounted /Volumes/DataRAID on /mnt/nfs then exported as nfs  
exports entry:
/mnt/nfs 10.0.0.0/255.255.255.0(rw,sync)

Anybody has any suggestions?

Thanks in advance.
CY