I sold my car in july 19 NOC has been issued on 10th July

I sold my car in july 19, NOC has been issued on 10th July 19 by Dwarka Transport authority to Firozpur (Punjab) Transport Authrity in favor of HARKIRAT SINGH mentioning reason as " Sold the Vehicle in other Registration Authority "

but Buyer has not get it transferred in his name , I sent him reminder on phone/ whatsapp and through registered letter but not getting response from him