Why couldn’t Darth Vader sense Leia was his daughter?

Passive Powers

In The Last Jedi, we learn Leia does use her Force abilities but not in the way you’d expect her to. She might be a Skywalker, but she’s not violent like her biological father tended to be. Instead, she takes over her birth mother and uses her intellect. Considering she only had a small amount of Jedi training from Luke, it’s safe to assume Leia’s abilities with the Force were passive.

To back up that passive powers existed, we need to jump back in time to Rogue One. In that film we meet Chirrut Îmwe, a blind member of the Guardians of the Whills. While he wasn’t Force Sensitive, he followed the teachings of the Force. He used it as a second set of eyes to help guide him and to fight in combat.

With Leia, her decision to use the Force for combat is what sets her apart from Luke and Anakin. They were like traffic lights in the Force and could be sensed from several parsecs off. Leia, however was nowhere near as powerful which is why she slipped under the radar as far as the Emperor and Vader were concerned.

Forgotten Memories?

Now, this theory might be a little… farfetched. What if the pain Anakin suffered when he became Vader caused him to forget what the Force feels like? Though, this is unlikely given that he recognized Ahsoka when they crossed paths.

While he might have recognized his former student, he might have forgotten how to sense newer signatures. His transition from Jedi to Sith was painful. He suffered every tragedy under the Tatooine Twin Suns. He lost his friend almost mentor Qui-Gon Jinn, his mother, Ahsoka left the Jedi Order,and than he loses Padmé and their child. All of this happens within the space of thirteen years. Events like this would leave anyone traumatized.

If Vader had memories as Anakin taking to Padmé about whether their child be Force Sensitive or not, he would choose to forget about them. There’s a possibility that the conversation escaped him because it was too painful to consider.

To think about what could have been in their future with their Force Sensitive child would be enough to drive anyone into a state of forgetfulness.