Why don't I see all my machines?

Hosts can appear or disappear from your network view for a number of reasons, for example, If a host is powered off or disconnected from the network prior to enumeration and its name is purged from name caches on the network, it will not be listed in the Network Pane.

Summary

Your network view is affected by:

Whether a machine was on the network during enumeration
"Enumerate" menu item or ...
Enumeration scheduling (File > Agent > Settings > General > Background Query Settings)
"Show only managed machines" display filtering (View > Options)
Current Master-Agent connection (File > Agent > Connection)
"Add Machine" menu item
MLF file use (Network > Import Machine List)
"Delete" menu item
Contents of network name caches (WINS, DNS)

Details

Your network view is affected by: