Are udaterui.exe and mctray.exe running on the affected systems? If not, check the agent log - are there any errors relating to udaterui?
(udaterui is the executable run in the context of the logged-on user that, amongst other things, displays the tray icon.)
Also - possibly a silly thing, but worth checking: make sure that the agent policy assigned to these machines is set to display the tray icon.
Joe, this is a dupe. You may delete it. The other one has all four attachments that I uploaded. I posted it because this one threw an error after uploading one attachment.