As I know, there are only four vans departing from Shenzhen Airport to Changping directly everyday. However, so many trains leaving for Changping Train Station of Dongguan. So, my suggestion is when you arrive at Shenzhen Airport, please take the shuttle bus Line 1 to Shenzhen Railway Station, where you could catch a train to arrive in Changping town on the spot in 30 minutes. This is also much faster than you take bus to Changping.

And, there is no direct bus to Chang'an.