Why my nfs client doesn't work?

[jqdkf@zju.edu.cn]

Hi,

I have set up an nfs server and it can be mounted by most nfs clients,
but there is one doesn't work. I use "rpcinfo -p ***.***.***.***" to
test that client, and it answers "Can't contact portmapper: Remote
System Error - connection refused". When I use "rpcinfo -p" on that
client, which is also an nfs server, it can show the information like
100000 2 tcp 111 portmapper. 
I can't figure out what is going wrong. the /etc/exports and
/etc/hosts.allow hosts.deny is right. Any help?

Zeng Nan