When Credit Agency will not remove paid off debt.

Hello everyone, I am hoping someone here can help me. My spouse and I are trying to apply for a home loan and one item on the credit report needed to be settled. We agreed with the collection agency to pay a certain amount, and they agreed in writing to remove it from our report after payment was made. We have two separate emails with this info and a receipt of payment.

Well, two credit agencies have accepted the letters as proof and removed the item, but one is refusing to do so, telling our mortgage person (who is helping us navigate this) that the collection agency needs to send them a letter specifically asking them to remove the account and it must have a letterhead (which the current letters already do). It took an act of Congress just to reach someone to settle the debt, and they are not returning our calls. In the meantime, our dispute is now in "research" at Equifax and we are stuck waiting, while the house we want could be sold to someone else.

Any ideas on what we can do now? Is this truly just a waiting game? Do I have to get the collection agency to write another letter? Any help would be greatly appreciated.