I have a large spawning area in the nether, where the only spawnable surfaces within a couple hundred blocks for mobs are on top of stone pressure plates. (See screen shot below.)
I haven't played in over a year, and my spawning area in the nether has NO mobs spawning in it at all. (See attached number of entities, the entities present are all minecarts, or chickens or players, no mobs.) My render distance is set to 35, difficulty is set to hard.
This question and answer seem to imply otherwise, but I can think of no other explanation for the paucity of mobs.