By the time I reply, you have figured it out. It appears that there is some building score requirement to even trigger the purge. Then depending on your location, there might be further complications like the purge not being able to find spawning locations (happens often when there is some water). It is a bit hit or miss with the purges.
I had some post about it some time ago, I am not sure whether the numbers are still accurate but may explain what you are experiencing: