I'm fairly sure the spells needed to transport from their world to ours and from ours to theirs are different, if only slightly, and that's why we've only seen this problem twice: when Aylia's father was trying to bring Aylia back to their world, and now when he's trying to do the same with Alassa. As for why Alassa can't just transport herself home, I think when someone transports you somewhere the way Aylia's father does, you need them to bring you back, as well. So she can't just do it herself.
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