DFS authetication issue.

I have 2 servers A and B and they are both stand alone 2003 servers.
I create a DFS link \\SERVER A\LINK which can redirect to \\SERVER B\SHARE.
There are 2 users in server A, administrator and userA.
And there are 2 users in server B, administrator and userB.
2 administrator accounts have the same password.

My question is when I logon server A with administrator account before I
access the \\SERVER A\LINK in a XP client, it's fine, I can access \\SERVER
B\SHARE. But when I use account userA, it won't shown me the logon dialog and
tell me wrong user and password. Why?