CRM 2013 Server side sync - delays in tracking emails

CRM 2013 SP1 UR3

We've noticed that some clients when they click Track in CRM seem to take about 20 minutes to show as tracked. It used to be the [View in CRM] button would be usable but its still greyed out now. The track button changes to untrack but the email isn't in CRM. We are using Exchange 2013, server side sync for emails in/outbound.

Tried reinstalling the client/clearing caches etc, any ideas what else can be checked?

Thanks 

Pete