Unified Inbox in Mail does not display all emails I have 4 different email accounts in Mail; therefore, there are 4 different inboxes. 

When I select the unified Inbox, I should see all emails listed from all accounts. But I don't. 

What could be preventing seeing all email in unified inbox? 3 email accounts are POP; 1 is MobileMe IMAP. When I control-click on unified Inbox, all email accounts are in the drop-down list. One of the POP accounts shows nothing in large box below that.

(Snow Leopard 10.6.8 with Mail version equivalent for Snow Leopard.)
My guess is that one of your clients is removing emails from the server - last time I used POP this was the default, and it was one of the best reasons for switching to IMAP. If any of your clients is doing this then those emails will not be appearing on your other clients. Have you checked this on each client that you access those servers with?

I know you didn't say you access those emails from multiple places but it's a common thing to do.