If Maleficent revoked the curse she places on Aurora, why does Aurora still prick her finger and fall into a sleep?
She didn't revoke it. She certainly tried her hardest, but it seems that once you've given someone a magical gift, revoking it is impossible (or at the very least requires more magic than Maleficent is able to muster).
The film's official novelisation gives us some good info about what's happening.
Feeling dread begin to build in the pit of her stomach, Maleficent spoke the words again, with still more passion. And then she repeated them. Again, and again, and again she spoke, mustering all her strength and willing all her magic to break the curse. The room began to shake as the massive amount of magic collected in the small space, but Maleficent went on, oblivious. All she could see was Aurora, sleeping the way she would forever if the curse could not be broken. Letting out one last cry, she threw her staff in the air and sent a huge burst of magic raining down over the room.
But it still didn’t touch Aurora.
Lowering her staff, Maleficent slowly left the room, her heart aching. She had done everything she could. Yet the curse, the one she had so foolishly called a gift, could not be undone. Which meant, one way or another, in a few short weeks, Aurora would prick her finger on a spinning wheel and never wake up.
She didn't revoke it.
She tried her damned best to because she had a change of heart, but she couldn't. Not because she wasn't strong enough or didn't have enough magic, or even because it was a 'magical gift'. But because of the wording of the curse: "This curse will last till the end of time. No power on Earth can change it."
This is actually evidenced from the same scene in the video. The curse she placed on Aurora manifests as a green cocoon of magic around her. Whereas when Maleficent is trying to revoke it, her magic is golden. (Side note: Maleficent's 'evil' magic seems to consistently manifest as green mist, whereas her 'good' magic is golden) Maleficent tries her hardest to revoke the curse, but in the end, her golden magic dies away, and the green cocoon remains around Aurora.
Obviously she was fighting her own curse as you can see from the golden and green mists clashing and the subsequent mini-explosion. But the gold then loses and disappears, with the green cocoon, the curse, still around Aurora. So the curse is still bound to her. And then in the background, the echo of Maleficent's curse can be heard: "...No power on Earth can change it..." So Maleficent intended that last sentence to prevent anyone from tampering with the curse, but it backfired because since she is on Earth, obviously she can't change it either.
TL;DR: Aurora still pricked her finger because Maleficent didn't revoke the curse. She couldn't anymore.