Account miss payment

I opened an account to boost my credit score, I paid my account every month but when checking my credit report it's says I missed one payment called customer service queried so many times and they said I never skipped payment they apologize system was updated incorrectly they will fix it, I sent all documents even proof of payment but still showing. I emailed experian nothing happened. I'm unable to get credit because of this nonsense. I would advice every one to stay away from that shop it's a ***** and their service is very bad wish it can be closed one day