I have just realised this has happened on a site I have developed too.
All forms should be sent to 2 email addresses.
Out of about 30 entries in the DB, 3 have never turned up as emails to either address (including spam folders) and 1has arrived at one but not the other.
Now we know we can check the dashboard, but this is a real pain as clients just cannot rely that all contacts are coming through.
Any suggestions?