Why would Exchange 2010 Delivery Reports show random results?

Looking for ideas on how to troubleshoot. MY Delivery Reports in ECP or Search-MessageTrackingReport in EMS will yeild what seems like random results for both internal an external email tracking. By random I mean only a small subset will show up. I've checked C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\MessageTracking and I've got 30 days of logs in there and the emails exist in the logs, and they are in my sent items, but only a random few will show in ECP or PowerShell. I'm running Exchange 2010 SP2 without Update 1, on Windows Server 2008 R2 with SP1, single server org with all roles but edge.

For example, I've sent 4 emails in last 24hr with the word papersave in the subject line with internal and external addresses in to/cc, but ECP Delivery Reports will only show one of the 4. PowerShell will yield the exact same result of 1 for 4 buy using Search-MessageTrackingReport -Identity myuser -Subject "papersave". This was happening before SP2 as well.