IS cannot connect the UM

Hi, everyone,
We’re using the webMethods version 9.12, there are several IS connected to the UM, and the document type and trigger can be used normally,We have been in normal operation for some time.
But yesterday we restarted the UM,there were some wrong situations:
1.Now we can’t allow the new IS to connect the UM, and the newly created document type and trigger can’t be activated.
2.IF we let those IS disconnect from the UM Alias , then the IS cannot reconnect the UM Alias again.
3.IF we restarted the IS,then the IS cannot reconnect the UM Alias again too.
Of all the status have the same error [ISS.0153.9051]
Error initializing UM session: com.pcbsys.nirvana.client.nRealmUnreachableException: Realm is currently not reachable:Realm was still unreachable After Max retry count - nSessionAttributes:conns=1/[nsp://ourIP:9000/].
Do anyone know what’s the reason? How can we solve it?