Dungeon Keeper 2 version 1.7 (which included the GOG and EA ORIGIN editions) comes with an issue that on many levels the correct creatures do not come from the portal anymore. For example you will no longer attract Dark Angels, Goblins, Black Knights, Fireflies during the campaign on certain levels. To fix this, download this archive and extract the files into your Dungeon Keeper II\Data\editor\maps folder. When prompted, say 'Yes to all' to overwrite the faulty files.

https://keeperklan.com/downloads.php?do=file&id=141

If You use Patch 1.7 make sure to download this for the best experience. There are Pros and Cons to both but overall you will be able to attract more creatures.