Negatively Affected Credit Rating

I had a contract with Nashua Mobile, I had cancelled it according to the policy, one month before expiration. After approximately 5 months i received a letter of demand from lawyers indicating that i need to settle my outstanding amount immediately. As far as i was concerned, there was nothing outstanding as i had cancelled the contract. I contacted Nashua, and soon realized that the lady whom i had requested cancellation from had not processed the cancellation. I sent the email to a consultant, and they assisted, i was told that the outstanding amount was cleared, and Nashua apologized for their service. Today i requested my credit report, and my Nashua contract has resulted in me being listed as a slow payer, and this has affected my credit rating. Why must I suffer due to somebodies inability to do their job?