I made a wither skeleton farm made by MCblockinvasion in a Minecraft SMP server. 
![The wither skeleton farm](https://i.imgur.com/CXQcjFM.png)

Here are the spawning pads:

![The spawning pads](https://i.imgur.com/zZsPa7H.png)

I slabbed the nether in a 128 radius around the farm.

![Slabbed nether](https://i.imgur.com/AwJ5fag.png)

There are no mobs spawning at the farm, and I am certain that I am far away enough for them to spawn. In this next picture, you can see that there is 15 mobs loaded, but no mobs are spawning in my farm anyways to reach the mob cap.

![15 mobs loaded](https://i.imgur.com/lnB8M31.png)

Keep in mind that even when nobody is online the server, the mobs still aren't spawning. Any help is appreciated.