Incoming mail not appearing in Outlook in box

The situation is a small office with 8 users using SBS2008 and Outlook 2010. The Outlook profiles are set up to use Cached Exchange Mode. The profiles are also set up (in Advanced tab) to open the other user's mailbox and access ALL the folders, this results is rather large offline folders (especially as the users keep large amounts of email in their email boxes and are reluctant to store it in PST files). A few weeks ago some users started to notice that they had received mail on their smartphones but not in their Outlook inbox, as time went by more users had the problem. I checked using Outlook web access and there was some email that was visible in OWA but not in the Outlook Inbox. If I disable the cached exchange mode it fixes the problem. I would however prefer to keep using the Cached Exchange Mode as some of the users have laptops that they take out of the office.  I am not sure if the "missing" mail will eventually be visible in Outlook I will do some further testing. It would be helpfull if I could get some advice on this problem and some suggestions on further testing.

Thanks

John