All collectors are visible in Workstation Investigator & WebView but some are missing under CEM Setup/Services/Service Configuration & EM Configuration.
The missing collectors are ones where 2 collectors are installed on the same server. In particular on the server running the Tim Collection Service (TCS):
[ERROR] [SessionPartition.Thread1] [Manager.com.timestock.tess.util.DbUtils] Execute failed for SQL DDL 'ALTER TABLE ts_us_sessions_map ADD PARTITION ts_us_sessions_map_20151209_13 VALUES LESS THAN (TIMESTAMP '2015-12-09 14:00:00 -5:00')'
java.sql.SQLException: ORA-14074: partition bound must collate higher than that of the last partition
For the missing collectors, the EM_HOME/config/internal/server/tess.internal.properties file contained a entity.guid that matched the corresponding value for the collector on the same server. That value must be unique for all EMs in the cluster.
The tess.internal.properties file with unique entity.guid is created when the EM first starts and connects to the APM DB. In this case the problem had been caused by duplication of the collecor1 EM installation on same server after collector1 had been started, so the tess.internal.properties had been inadvertently duplicated to the collector2 EM installation.
Per TEC1248765 ("Seeing this error in multiple collector logs: Unable to create partition for") :
Stop each collector which has the problem:
Delete the file EM_HOME/config/internal/server/tess.internal.properties
Restart the collector.