I am having same issue of wther skeletons not respawning. I will find and kill 3 or 4 after entering a new nether fortess and not get any skulls, then run around for an hour and more killing blazes, ghasts and cubes, and not finding any more wither skeletons. In my desparation to obtain a skull I am attempting to attract a charged creeper through a portal and blowing up a trapped wither skeleton. This is very time consuming and requires great luck. There should be an acheivement for this!
Benito
I am having same issue of wther skeletons not respawning. I will find and kill 3 or 4 after entering a new nether fortess and not get any skulls, then run around for an hour and more killing blazes, ghasts and cubes, and not finding any more wither skeletons. In my desparation to obtain a skull I am attempting to attract a charged creeper through a portal and blowing up a trapped wither skeleton. This is very time consuming and requires great luck. There should be an acheivement for this!