All Inboxes not showing message count

I recently noticed that the message count is not appearing for All Inboxes consistantly.

If I click on individual mailboxes it show (# unread / total) e.g. (0/13).
If I click on a mailbox with no messages is shows nothing.
If I click on All Inboxes is sometimes shows the message count and sometimes does not. Have not been able to determine if this is something I am doing or a bug.

Any ideas?