Ash tried to experiment using her fear-based spell to form a connection between herself and the balls of light to test if she could sap away some of the Life Energy before it died but she soon found out that it would not work.
Certain conditions needed to be met and one of them seemed to be that the form had to be completed.
It was possible that the form needed to be similar to an animal or human for her to be able to extract their Life Energy before their death.
Her form needed to be a living one for her to be able to draw away their Life Energy similar to how humans and animals could be targeted by her.
Those animals and humans that she had taken Life Energy from would have their form taken by her and she could take on those forms.
The forms of Evil Spirits or Spirits were just too incompatible for her to be able to transform into them it seemed.