So I finally got to the point that I thought having a small farm would be helpful. I think I had about 4 each of chickens, sheep, cows and pigs. Two times I tried to set this up and each time within about an hour, the animals would disappear. Now I have had 2 pigs, a chicken and a cow for a while before this and no problems. It was only when I brought the others in that this started. Is there a glitch? Thanks.
No glitch, it's a server feature to prevent lag. When the pasive mods reach a certain number inside a chunk, the mobs will be despawned.