Product: Mortgage

Greetings All, Hopefully everyone is having a great week ... sorry to be a bother. 
I had been using XXXX since XX/XX/XXXX and had direct withdrawal configured for my account. At some point XXXX sold the mortgage to NationStar Mortgage, but neither bank provided notification of the mortgage transfer until XX/XX/XXXX - when I received a collection notice from Nationstar mortgage. I immediately called both banks and arranged payments for the amount due and configured automatic payment as the money was just sitting in an account waiting. 
The complaint - when talking to nationstar, I asked if this would affect my credit rating and they told me no, as there was a grace period. However, when I went to apply for a loan for Solar panels for our house, I was declined and when I checked my credit, I found it had dropped over a XXXX points due to NationStar marking it as over 60 days past due. So without providing 15 days written notice from the seller ( and never from the purchaser ), they transferred ownership of the mortgage and then negatively affected my credit rating ( which means I will not be able to refinance the variable rate mortgage they now own ). Nationstar had my address as they could send me a collection notice immediately when they thought I was past the 60 day grace period. However as my due date should be XX/XX/XXXX, I actually was within the grace period. 

My original mortgage was due on the XXXX of every month. When Nationstar took over the mortgage, they changed the due date to the XXXX of each month - which I told them was not legal as it changed the terms of the mortgage. They did n't appear to understand. I also asked for the transfer of mortgage ownership information ( 3 times so far ) and have yet to received it. And they keep getting my name backwards - not that big a deal, but a little irritating when you have to be transferred to different department each time so they can " correct it '' again. 

I contacted Nationstar and asked why I had been lied to and why credit was negatively affected. They sent me a pdf in email stating they had never put a negative credit mark on my record - however all the credit agencies say otherwise. So I responded back with screenshots from XXXX and have been waiting since XX/XX/XXXX for a response. 

I also filed a complaint against XXXX - who responded saying they attempted to send a notice, but got the change of address I had filed with the post office showing my new address .... so they had the new address, they just did n't use it. They provided a document for XXXX that was reprinted and had no date in it, just a stamp done after the fact on the top stating XX/XX/XXXX - which would not have been sufficient even if mailed to provide 15 days written notice prior to a transfer if the mortgage due date was XX/XX/XXXX. It would have been fine for a transfer on the due date ( XX/XX/XXXX ). XXXX provided no record of sending the document. SInce XXXX is not the entity that negatively impacted my credit, I 'm ok leaving it there. 

I want to know why my credit was negatively impacted when the mortgage company said it would not be and also why they did not contact me to tell me of the ownership change if they had my address - which they used to send me a collection notice as soon as it was in their best interest. 

thanks, XXXX