cannot subscribe a node that has been published

1. i've create a node from the server by  user 'u1@myserver':

          <iq to='myserver' type='set'  id='create'>

               <pubsub xmlns='http://jabber.org/protocol/pubsub'>

                    <create/>

               </pubsub>

          </iq>

and the server responses:

          <iq XXX>

               <pubsub XXX>

                    <create node='7YF9mthSxM46bV9'/>

               </pubsub>

          </iq>

2.the,i changed to user 'u2@myserver' to subscribe the node i've been created:

          <iq to='myserver'   type='set'   id='sub'>

               <pubsub xmlns='http://jabber.org/protocol/pubsub'>

                    <subscribe node ='7YF9mthSxM46bV9' jid='u2@myserver/abc'/>

               </pubsub>

          </iq>

but the server response with an error '404':

<errorcode='404'type='cancel'>

<item-not-foundxmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>

</error>

can anyone explain where the problem is , thx so much