Managed system config error - Assign diagnostic agent

Hi ,

I have installed diagnostic agent on eight app servers successfully with no error. In managed system config for assign diagnostic agent step, I could see all of them on the agent assign tab and when I try assign their host agent with server name , only five server names are appearing there. Even in the SLD agent candidates all the app server are viewable and two are showing the below errors.

1. Access denied by the managing system. check the user/password

2. Agent registration failed ( connection refused by managing system)

How to resolve the error and why the three are missing..

Please help.

Thanks in advance