Also, sometimes the animation sticks. In this case, you can either turn ops off and then back on again or switch to another farm and back to your main ranch again, or just refresh the game window. One of those will work. Everybody has trouble w/sticking animations from time to time.
I have found that even while auto is turned on, unless you are actually watching the buffalo, i.e. in the water ranch, then the buffalo will not work. So if you are on your home ranch, then the buffalo will not eat, even whilst the auto is on.