Fix deragatory credit item?

Hello, I am looking to get a mortgage and b”h I have the down payment for the house. My problem is that last year at some point, my student loans were transferred from one company to another company called Nelnet. During the transfer I did not receive any notice and although they left many phone messages, it was for a number I was no longer using. By the time I got in touch with them, they had reported the payment as late. Despite my adamant demand that it be removed, they said that there’s nothing they can do as they payment was late and will remain reported as such.

It gets worse, as the structure of the loan is split into 5 different accounts. As a result, the credit report is showing that at that time I missed a payment on 5 separate credit lines! Even though all my other credit items show a great payment history including on cars and credit cards, as you can imagine this is killing my FICO score which is what the mortgage banks rely on.

Does anybody know anyone reliable (lawyer maybe?) that can get this removed quickly? Any other suggestions??

Thanks.