Mob farms do not seem to be working properly, and seem to be spawning virtually nothing in most instances. 

For this, I have screenshots of what I've tried:

#1 standard, 2 high spawner, with a water drop for the killing blow

After an hour of waiting AFK, nothing spawned. I took in account spawn limits, and did my best to light up any caves in the area. Still, nothing spawned.

#2

My next attempt was to raise it from the ground. These screenshots should be self-evident, but it appears, that even with 4 blocks above the floor, mobs prioritise spawning on a flat open plain, with no sides, or something with a more rigid bottom. Instead of spawning within the spawner, the spawned on top of it. When I added layers to see if walls were the problem, they again, went to the next layer.

Here's a series of screenshots to demonstrate:

Something isn't working right. Not sure what it is.