Outlook items do not sync with zimbra webmail

Hello,
Not sure what I am doing wrong but on a few accounts we have the changes made in outlook do not reflect in zimbra webmail. For instance if one of these users reads a message the message is still shown as unread in zimbra, or if a message is moved to a different folder it is not being moved. Also deleted emails are not being removed from the server as well. I've looked at several accounts to see if there were any differences in the outlook setup or the user account but can find none. Any suggestions?