I know what you’re missing because the same thing happened to me when I started messing with RetroPie. And you’ll hate me cause it’s annoying ;)
You have to press the PS button in the middle of the controller (after everything is plugged in ofcourse) and it magically starts working.
Other pitfalls:
* Controller turns on PlayStation and connects to that, using the pi to charge only &_&
* Controller loses sync with PlayStation but doesn’t power off doing a minute long search cycle (also not letting you use it on the pi in the mean while), you’ll notice this as all the LED will be blinking.
* The power you’re supplying to PI is inadequate. It’s a long shot, but my $hit adapter didn’t provide the full 2A and the pi wouldn’t recognize some USB devices (but it does with others).