Neutral mobs disappear when loading a world or switching resource packs

When loading up a survival world (normal difficulty) neutral mobs, in this case, cows, sheep, and pigs sometimes do not show up. These mobs are kept in farms, so I am not referring to mobs spawning in random terrain. Upon reloading the world or switching to a resource pack, some of the mobs reappear. The amount of mobs that reappear/disappear seems to be inconsistent.