Haha, anyways...
It's hard getting a person to follow you. Usually my pal Liam Payne does follow sprees where he follows random fans on his followers list. It's rare though when all of the lads follow their fans. They usually follow fans whenever they are bored, having nothing to do which like I mentioned before, is RARE.
You could always ask a person who they FOLLOWED to ask them to follow you. Not a celeb, but a regular ordinary directioner.
Secondhand Serenade followed me. (The guy who sings "Fall For You") He follows everyone though. It took me by surprise. I was like "WOAH! No way! Is this for real?"
Try tweeting something funny/sweet to them when they are online so they could see it. It would be hard though.
I have a handful that follow me, but some follow you 'cause they are promoting something and aren't interested in you, while others follow you because they found you interesting in some way. It must be understood that they are busy and one must respect that, in other words don't stalk them and say dumb things. Also, they may not follow you forever.