Rahkhun

I have a gastly (now haunter) that should from what i know have learned shadow ball by now. However it has not and instead tried to learn payback. When does it gain the move? and are there any other major differences in learned moves in reborn?

As the title says I'm looking for someone to trade me up a gastly if anyone has a spare going? Got my mystery egg last night and ended up heart broken it's a shroomish instead ;_;