This seems to be a problem only on the multiplayer server. I tested spawning above the nether in a creative single player world and they spawned without a problem. On our multiplayer server, though, we built the Gold Pan 3 in 1 farm (https://www.youtube.com/watch?v=z-h0goQZvso) noticed that we aren't getting a single mob to spawn at all. There were only two of us on at the time of testing, both of us on the wait pad over 25 blocks away from the farm. Being as how it worked on my single player map, it seems to only be affecting either multiplayer servers in general, or perhaps it's just our server?