Does it say "Attention Required" ? I had that problem along with my calendar and contacts. After about a week of struggling I finally had to uninstall activesync and reinstall. If someone knows the answer to this problem I would like to know in case it happens again.
You need to provide more information, does ActiveSync tell you there's an "Unresolved Item" ? Does it just skip the email? Does it crash? What happens, what doesn't happen?