Hostile Mobs Not Spawning Above A Certain Block Height

The frustrating problem i have encountered is the spawning algorithm set for hostile mobs. I just completed my first mob farm created after the Automatic Mob Farm Flush O' Matic by Snocrash. I created this farm in console edition and it works great. But the two serious problems i was faced with was build height and mobs not spawning. I built my farm over an ocean and to build height, which is only 64 blocks above sea level. But with everything complete mobs are not spawning in the spawn rooms at all. The light levels are 0-1, being impossible to see inside, and with a cealing height of 4 blocks there are still not a single mob spawning inside. The mobs seem to not spawn at a certian height. With the lowest spawn room at about 45 blocks and the top at about 60 blocks high. The ocean is very deep so caves underneath are minimal.