Sim swop charges

I got a micro sim card in about April 2015 because I bought a new phone that needed one. I then paid for the sim swop. Now in September the sim card stops working. I was then told by customer care, after the second call, that because I'm a contract customer I will have to pay again and do another sim swop. I would like to know why I must pay for a defective sim card thats made for Cell c. Am i suppose to pay for a sim swop every 5 months because your sim card quality is inferior to other networks? I have never had any problems with vodacom in the past. I am not happy with the service received from cell c.