Some of the SQL Server 2012 are not getting monitored in SCOM 2012 SP1.( basics monitoring is happening, expect SQL role)

Found that all the SQL servers are getting monitored, expect few servers which are having SQL server 2012 role.

proxy is enabled for these servers, when I checked discovered inventory for SQL 2012 database unable to find these servers in that list.

and other servers having SQL Server 2012 monitoring properly issue is with only few servers.