Mob spawns above the nether

I'm running my own server and have noticed this problem on a past server. Before building a gold farm on top of the nether, I first built a platform approximately 10 x 10 blocks at block level 200. Nothing at all spawned. I then recreated this experiment on a single player map. As you can see from the picture, plenty of spawns. I then started a server on my personal computer and connected to localhost. Again, no spawns. This seems to be an issue affecting minecraft servers. Attached also is our server properties (with the seed deleted of course) so you can see our render distance is set to 10 so that is not a viable fix.

Also, on the single player test, they started spawning immediately. In the server test, I let it idle for half an hour with not a single spawn.

#Minecraft server properties
#Sat Jul 11 08:32:15 EDT 2015
spawn-protection=0
max-tick-time=60000
query.port=30123
generator-settings=
force-gamemode=false
allow-nether=true
gamemode=0
broadcast-console-to-ops=true
enable-query=true
player-idle-timeout=0
difficulty=3
spawn-monsters=true
op-permission-level=4
resource-pack-hash=
announce-player-achievements=true
pvp=true
snooper-enabled=true
level-type=DEFAULT
hardcore=false
enable-command-block=true
max-players=18
network-compression-threshold=256
max-world-size=29999984
server-port=30123
debug=false
server-ip=192.99.148.48
spawn-npcs=true
allow-flight=false
level-name=Batlantis
view-distance=10
hellworld=true
resource-pack=
spawn-animals=true
white-list=true
generate-structures=true
online-mode=true
max-build-height=256
level-seed=***Censored***
use-native-transport=true
motd=Welcome to Batlantis
enable-rcon=false