Why was Voldemort unaware that Harry was also a horcrux?

TLDR version: desensitization. It happened seven times already.

I don't know if it's the in-canon answer, but there's a phenomenon called desensitization, which is when something shocking or painful happens so many times you forget that you're feeling it. For example, some soldiers who have put up with a lot of violence will simply put up with it. Know it's there, recognize it, but shock factor is no longer there. It happens with physical pain too, as your brain pretty much tells the hurt limb "yes I know you're hurt shut up" if it's been hurting for so long.

I wouldn't know what a torn soul is like, but if it's been ripped seven times he can probably put up with it again. (is that the right number?) This theory is more logical if the pain associated with a torn soul is a constant ache... Which seems logical to me...

Okay, so he's used to the pain. But why didn't he notice the magic happen? Well... I don't know if there's a light show that happens when a horcrux is made, like with other spells or with the Dementor's Kiss when it shows the soul, but even if there is, it happened at the same time as the killer curse, so it would have been covered up well by that green light.

Oh and also he was distracted by DYING from Harry's mother's sacrifice.

So, he didn't notice that he made a horcrux. If he didn't know he made one, how would he know that the soul piece stuck to Harry in particular?

A couple of things to clear up here.

1. Harry was not a Horcrux
   At least not in the traditional sense. Voldemort never intended to make Harry a Horcrux, but basically the killing curse rebounded, split his soul (see point 2 for the why) and the soul piece just "latched-on" to the only living vessel - Harry.
   Dumbledore either didn't know any better, or he just simplified it (for Harry or for the readers) when he told Harry that he was a Horcrux.

2. Voldemort did intend to make a Horcrux that night
   This may explain why his soul was ripped apart when the curse rebounded. There's some strange and unknown ritual to perform when creating a Horcrux, this involves:

   • Performing a spell or curse
   • Committing murder
   • Transferring the soul piece into a container of some sort

   The guess is that Voldemort had performed these rituals, but unfortunately got rudely interrupted by Harry's head rebounding the Avada Kedavra. The detached piece of soul had nowhere to go but to latch itself onto Harry.

3. Voldemort didn't feel his Horcruxes being destroyed because he was so E-ville

   "I believe not. I believe that Voldemort is now so immersed in evil, and these crucial parts of himself have been detached for so long, he does not feel as we do. Perhaps at the point of death he might be aware of his loss . . ." - Prof. Dumbledore
   _{-Harry Potter and The Half-Blood Prince, Chapter Twenty-Three (Horcruxes).}

No one, not even Voldemort himself knew what he was; let alone know that Harry was deeply connected to him in this way.

As an alternative theory, Voldemort couldn't figure out that Harry was in every sense of the word his horcrux, because Voldemort became Harry ' s horcrux when he took Harry's blood to recreate his body with Lily's protection.

Since this results in both of them as Horcrux for each other, while Voldemort was unaware of Harry's status, he knew there was some connection. This caused Dumbledore to charge Snape with trying to teach Harry how to block Voldy out of his head. Voldemort likely chalked it up to his taking Harry's blood, not Harry taking part of his soul. Since the magic was performed differently from the regular Horcrux spell, Voldemort had no reason to believe there was any soul sharing.

According to J. K. Rowling in a Pottercast, there's a spell involved. I'm guessing Voldemort never got around to casting the spell (or if he did, it probably wasn't on Harry), so Voldemort probably never suspected Harry as a Horcrux.