Problem in transferring the ownership of Bike

Hi,

I sold my bike in a shop buys used bikes. In turn they sold it to someone. The new owner of the bike is not transferring the bike onto his name, and stopped receiving my calls altogether. Even though its been a year, I am unable to get the bike transferred to the new owner, as he is not willing to come to the RTA. When, I inquired in the RTA for a solution to this problem, they said, they couldn't do anything unless the new owner gets the bike transferred to his name. Can anyone suggest what can be done to this problem?

Note:- I have the scanned copy of the bill, that is issued by the shop when the bike is sold.

-Regards,
Naveen.