If you in ACE Agent are working with a chat or an email and want another contact from a queue you must first Park the contact in progress and then Activate ACE Agent again.
Park or Ctrl Shift P parks current contact and you end up in Paused status.
You see a parked contact in Contacts in progress. Double-click to retrieve a parked contact.
If call arrives at your extension the current email/chat is automatically parked and is reactivated automatically when the call is ended.