same happened with me when i bought one. locks tend to wear. goto lock smith and open up the lock. he will weld a bronze aka "Peetal" plate to ur existing worn out lock plate of that shape. And ur lock will be like new. Rs 300 charges, 15 min Job
I have done it for one of my doors now will do for the rest three. U have to find a good locksmith for this job or give him this idea to cast the plate and weld it to the existing worn out plate