I am having the same issue. Are you running RSD 220.127.116.11? I never had issues when I was running 4.5. Now random RSDs turn passive, and in general the sensors are not sending much of their info over to the server. Quite frustrating since we use the sensors for much of our reporting here (for instance how many printers we have, etc).
When a sensor turns passive I usually send a job over to reset the RSD service, and if that does not work I reboot the problematic DHCP servers.
If you find a magical solution, please post it! I will do the same.
Still running 4.5 (v 137) on our sites.
We only recently set all our sensors to active instead of the default setting of 2 per subnet. Wasn't my call to enable that setting.
A possible fix: I noticed many of the sensors from one network were also broadcasting IPV6. Had an admin disable IPV6 on that machine. He then restarted the RSD service and the sensor went active and stayed there. Interesting, the only ones broadcasting IPV6 and going passive were workstations. I have servers also going passive that are NOT using IPV6.
More info as I get it.