Minimally, the rear half of the car does have independent controls, so I assume you tried those first: if not, expose the full AC user interface (touch the icon shaped like a fan blade), locate the rear section controls if they exist (for a six-seater they should appear below the front seating controls), and manually increase the fan setting to 11 and decrease the temperature to LO
for just the rear seats.
That should at least give you an idea of what the car thinks it is capable of.
Note that manually adjusting rear AC settings will turn off the "Auto" feature for the rear section. So whatever you set will be remembered until somehow Auto resets it all. Also if you are testing with Auto selected, make sure there is someone sitting down on a rear seat if you want Auto to function as intended; if the car doesn't sense any weight there, the Auto setting disables AC in the back altogether.
It can take time to cool down all the ductwork, so don't conclude 'they don't work' until after they've had at least a few minutes to run. Also recall 'Range Mode' reduces peak climate power by design, so turn that off if necessary.
(This is all in the manual. Climate control starts on page 139:
https://www.tesla.com/sites/default/files/model_x_owners_manual_north_america_en.pdf )
If you've done all the normal stuff and still don't like the result, well, many have concluded rear AC in the X was never adequately provisioned even when functioning as designed.
Also consider three other possibilities:
1) Make super sure it's actually a 6-seater, not a 5-seater. The 6- and 7- seat X's came with two AC compressors, the 5-seaters only had one. And I believe in the latest firmware's user interface, there *are no* dedicated rear controls if the car thinks it is a 5-seater. Regardless you won't get as much rear cooling if it's actually a 5-seater.
2) It's fairly common for a) ductwork to become disconnected on one side or another or b) for an electrical malfunction (loose connection or failed compressor) to reduce rear AC. Finding stuff like that requires a more involved troubleshooting process obviously.
3) There's always the traditional reason AC loses its zip over time, the refrigerant may need to be recharged. But honestly I haven't heard that come up very often, compared to these other issues.
[NB Your original post does stipulate pillar vents, but just for the sake of completeness, I'll mention here that the two vents under the front-seat armrest are in fact controlled by the front AC controls, despite the fact that they obviously point rearward.]
So there's a prompt, non-authoritative response so you can get started. Others doubtless have more insights:
Ok crowd, what'd I miss?