Emails are not pushed immediately to Cliet

Hi

There is an Exchange 2010 Server (SBS Installation) and 10 mailboxes. All clients are using Outlook 2010.
About 5 users have configured multiple accounts within their Outlook. Outlook is used in cached mode.

Problem:
New emails are not pushed to the Client immediately, even not by clicking the Send/Receive button. The only way is to close and to restart Outlook, to use reconnect in the "Connection Status", or to wait. After about 20 or 30 minutes new email are then finally pushed to the Client. By the way, the connection status is connected all the time.

If I disable Cache mode, everything work perfectly, but these are mobile users, and they need cache mode.

Any suggestions about that issue?

Thanks in advance and regards
Peter