I have had several users complain about this issue when using the AJAX (not sure about standard html) client. If they are in the inbox and move an email to a different folder under Inbox, no new emails will show up. They have to either click on the get mail button or re-click on the Inbox folder at the top to see their new emails. They can be waiting for hours before realizing that they are not getting new emails.
I tried searching and did not find anything similar. Hopefully there is some sort of fix for this.
Release zcs-6.0.7_GA_2473.F7.20100616200740 FOSS edition