The same thing happened to me. My region was at 25, I killed a couple of machines and a base spawned. I did that again after I destroyed that base and now nothings happening.
+1 from PC as well. Played a region from level 3 to 15 with no bases spawning at all (except the first one).
Tenebris also confirmed no bases at all for 10+ hours. Most definitely a bug here.