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[Star Wars] What is the canonical reason that Anakin & Obi wan didn’t go back and rescue Schmi from slavery, after the events of the Phantom Menace?

If memory serves, Qui-gon jinn just didn’t have enough money on him at the time to buy both slaves, & the plan was to go get some more from the Jedi council & return for her later.
It’s not like they were in the full swing of the clone wars yet either.
gpt
(From the canon book Queen's Shadow) 

Qui-Gon probably would have, but he was the only one aside from Anakin. And his death closed one of the best avenues of rescue. 

The Jedi simply didn't care. Nor did the Republic feel like messing with affairs in Hutt space. Even a minor thing like taking a slave from a backwater like Tatooine would cause problems with the Hutt cartels that the Republic at the time was ill suited to handle. 

Obi Wan was too strictly adherent to the Jedi code at the time to allow Anakin that kind of personal attachment. All Jedi in one way or another is cut off from their families. It's supposed to be how they operate and how they remain neutral in a constantly changing galaxy. He and the other members of the order constantly told Anakin this, and he pretended to accept it. (although we see how he really feels in Episode 2)

That being said, Padme had some aspirations to do exactly that when her time as Queen was done. Not only to rescue Shmi, but to free more slaves from Hutt control. It was Palpatine who convinced her not to go, mostly because of the potential catastrophic political consequences.

Instead she sent her handmaiden to do the job. Unfortunately, the handmaiden reached the planet almost exactly after Shmi had joined the Lars family. She did not get any solid leads on Shmi's location, unfortunately, and considered the task a failure.