Not a single skeleton skull, wither or normal, after using it to kill 128 skeletons.

Works just fine for me.

Are there certain conditions for it to work? I just used a couple of stacks of spawn eggs in the overworld, in my superflat test world, at night.

There's no special conditions for it. It should just work. Are you using the latest release? Any other mods present? Using regular vanilla skeletons and not SpecialMobs?

Don't have Special Mobs or anything else that changes vanilla skeletons. Latest release. Clearly, it SHOULD be working...will test more.

it doesn't work for me either

Once again, it works just fine for me. Give me details like what version you're using, what other mods do you have installed instead of "It dun work", please.