This can happen for a couple of reasons.
First, the Daily Quarantine Report does not get sent out if nothing was quarantined in the last 24 hours. So it's possible that you just haven't received any spam or viruses in the last 24 hours. To confirm this, login to the MXGuardian control panel and check the date & time of the most recent message on the Quarantine tab.
Second, if you have any other spam-filtering products installed on your mail server or mail application, the quarantine report may be getting blocked because it contains content that other filters may determine is spam. The solution is to turn off your other mail filtering applications and rely solely on MXGuardian, or add email@example.com to your other filter's whitelist.