When you said "there is no graph", I guess you meant that the line of the graph is at a permanent zero.
I don't use mail shield at all, so I cannot confirm for you this behaviour. Maybe someone else can.
The question is if the mails are really scanned and the problem is the counter, or the mails are not scanned at all, (hence the counter is correct, but the shield is not performing any activity, even if it is activated).