Spawning mechanics are very different on Bedrock. Is that the problem you're having? They're not spawning? There's also a bug where Looting enchantment doesn't increase the chance of drops. That could be a problem too if you're going for those skulls.
It's all in the spawning mechanics, don't think you'll get a wither skelly farm to work. There's a lot of farms that you can rework in better together minecraft... but there's some that just aren't possible.