1. Why some of my SQL Servers are missing in "Discover from Network" page
Reason1 : SQL Servers and SQLDBManager Plus are in different domains
Solution :
If the SQL Servers and SQLDBManager Plus are in different domains, you
need to add those SQL Servers manually into SQLDBManager Plus using
"Add SQL Server Manually" or "Import from CSV" page.
Reason2 : The SQL Servers were down when SQLDBManager Plus was started
Solution :
SQLDBManager Plus will discover all the SQL servers in the network only
while server start up and it will not do discovery again until it is
restarted. During discovery, only SQL Servers that were up will be
discovered. So you can restart SQLDBManager Plus to discover these SQL
servers or add them manually.