If you have a database that starts with the string local - mongotop ignores it. It also ignores any collection that ends in namespaces.
To reproduce, create a database localized and insert into any collection. Also create a database, test and insert into a collection named namespaces. mongotop reports neither when run.
shell
(mongod-2.8.0-rc1) admin> use localized switched to db localized (mongod-2.8.0-rc1) localized> db.a.insert({}) Inserted 1 record(s) in 74ms WriteResult({ "nInserted": 1 }) (mongod-2.8.0-rc1) localized> use test switched to db test (mongod-2.8.0-rc0) test> db.namespaces.insert({}) Inserted 1 record(s) in 67ms WriteResult({ "nInserted": 1 })
2.6
mongo-tools$ ~/mongodb/2.6.5/mongotop connected to: 127.0.0.1 ns total read write 2014-11-24T19:45:49 test.namespaces 0ms 0ms 0ms localized.a 0ms 0ms 0ms local.system.replset 0ms 0ms 0ms local.startup_log 0ms 0ms 0ms admin.system.version 0ms 0ms 0ms admin.system.roles 0ms 0ms 0ms
2.8.0-rc1
mongo-tools$ mongotop ns total read write 2014-11-24T14:46:50-05:00 admin.system.version 0ms 0ms 0ms admin.system.roles 0ms 0ms 0ms