I set up a monitor script: https://github.com/DoctorLai/SteemWitnessAutoSwitch to send an email notification to me when a block is missed. Below is the email notifications for the recent missing blocks:
As you can see, on average, a block is missed every few days. Sometimes more frequently and some times less frequent. A block may be missed due to network latencies, or simply caused by your clocktime not accurately up-to-date.