On the level 7 "Heart Attack" I have built 6 Garrisons. However, only two Augres ever came in.
You should get 3 Augres out of it, if you have built other rooms, meanwhile, they won't spawn. It takes sometime until the next unit will sapwn. Or perhaps you reached the Max Population.
Here's the Link for the Spawn Rules.
https://forum.brightrockgames.com/threads/spawn-rules.9727/
If it's really an game failure, please upload your output_log, where you played this level and savefile.
Closing due to lack of customer response.
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