What happens when you capture all of team plasma's sages in Pokemon black?
Looker appears and tells you that there was a sighting of someone riding a large black Pokemon. Then he just leaves.
It all has to do with the right timing. I also think it has to do with the number of Pokemon you've caught...if you've caught more, you have a higher chance of getting a critical capture. On my Black version, I've caught almost all of the Unova Pokemon, so I get critical captures all the time. I hope this helps you!