As a soul creature, even the Undead must follow the laws of the soul, acquiring a new body requires adaptation!
Just like before when those Mutant Creatures were transformed into witches, they needed time to adapt to their new bodies, the Undead themselves are summoned from human skeletons, so naturally, they adapt to the original skeleton body.
The soul needs time to adapt to the body!
Now trying to make a soul controlling a skeleton immediately adapt to a metal lump and complete control is certainly impossible!"
The main reason she couldn't solve it in a day before was because she was looking in the wrong direction, always suspecting if it was a gene problem.
Often as long as the problem can be found, solving it becomes much easier.
It's merely a soul compatibility issue, so directly enhancing the corresponding ability will do.