Have you tried to force it to do an immediate status check? 

No, since the status is updating on schedule every 15 minutes. And these mailboxes have been "NA" for 3 days. However, I just tried forcing it as well as running journalmailboxmanager.exe /regserver and still shows NA.

Restart the services. Make sure the VSa is local admin, if needed, reconfigure monitoringwebapp.

If the services are restarted, you should see 10 counters instead of 26 (or 24?) That will last approx 24hrs, then will show them again.

Possible corruption of the monitoringdb?

Oh wow, didn't realize it was 3 days.  :)

hmmm...have you seen the attached?

It has this:
Troubleshooting journal mailbox monitoring
If there is a specific problem with Operations Manager’s journal mailbox monitoring failing to return any data, there may be a problem with Journal Mailbox Manager registration.
To investigate this, use DTrace on MonitoringAgent.exe and examine the DTrace log for entries similar to the following:
JournalMailboxManager with CLSID <GUID> not registered If you see entries like this, register the Journal Mailbox Manager as follows.
To register the Journal Mailbox Manager
1    Open a Command Prompt window.
2    Navigate to the Enterprise Vault installation folder.
3    Enter the command: JournalMailboxManager.exe /regserver

... all the steps mentioned above. Guess it's time to open a case....

have you tried clearing the statistics on the databases?