Wither Skeleton are NOT Spawning

Thank you for your report!
However, this issue has been closed as Cannot Reproduce.

We were unable to reproduce the problem you are describing in the latest version. If your world is an older one, you might need to explore find new Nether Fortresses, due to a fix in 0.17.0.1:

Good news: guardians, wither skeletons & blazes respawn naturally again when switching from peaceful game mode to easy/normal/hard. Bad news: they will only do this in newly generated structures, not ones that have already been discovered.

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I just tried that and the Wither Skeletons are still not spawning at all.

They'll only spawn in newer chunks. Otherwise, you'll have to make a new world.

I am having this problem in 1.0.8.

I created the world in 1.0. I found my first nether fortress in 1.0.3. No wither skeletons.
I found a new nether fortress in 1.0.8. No wither skeletons.

Am I supposed to switch to peaceful, then switch to normal (or some other difficulty), and then find a new nether fortress?