There are a few in game animals that occasionally will not be fed for some reason.
Insure that there is a flat tile next to all fence pieces, and at least a 3x3 area inside the exhibit in front of the exhibit gate, as well as at least 3 tiles for the zookeeper to walk on outside the exhibit to reset their scan.
Sometimes though, even if these things are done, zookeepers will fail to feed the animals.
I have noticed this with pandas and sea otters, and I have no idea how to fix it other than rebuild the exhibit or move it to another location.