When you get this message, is your exhibit suitability at least 90 or higher? If not, then there are things you can do to improve that suitability. Maybe add more rocks or foliage. And with your foliage, if it is a quarter-tile foliage, make sure you put all 4 in one game tile. It may not look as natural as some may like, but it will help with suitability. Some animals may require elevation that may not be specified on the zookeeper's recommendations. I've had this happen before. You may want to play with that too. And, of course, if you get the red unhappy face, that's not what they want lol.