Hi Vijay,
Please leave the notification disabled, run just the PCo Agent instance used by MII for Queryies, and execute the MII query multiple times and see if you receive any values for any of the queries. It appears that the Wonderware OPC server is not handling an un-subscribed OPC Group correctly.
Also, do you have an OPC Client that you can connect to the FSGateway OPC Server; the Wonderware Client is not OPC but Suitelink, I believe. You can try the Rockwell OPC Test Client if it is installed.
Regards, Steve