5 months query

Good day, I have never had any bad experience from wonga until last year, I defaulted 2 installments due to that I lost my job and I was handed over to blake and associates then I was offered 50% discount to settle my loan and then i paid last year in November and i was promised that my account will be closed, as i kept checking on their portal my account was never closed and it kept on accumulating interest, when i called blake and associates they confirmed that my loan was settled and they have sent an email to wonga to close my account, as soon as i called wonga they requested proof of payment and I forwarded it to them, while i was still waiting for feedback they handed over my account to nimble group and i explained everything to nimble group and they requested me to send paid up letter from wonga..and then i sent it to nimble group and they said my query was escalated since then my account is still open and my credit score dropped from 620 to 584,why is wonga taking more than 5 months to resolve a query while i have a paid up letter that i received direct from them.