Adoption Based Greencard

I submitted all the documents for my adopted son In October 2018, He fingerprints was done a month later, but since then we haven't heard anything. The documents were send to Lee Missouri. My husband and I we are citizen and my son is a minor. I don't know what to do? why this is taking so long. No one give you and answer. Can some one help me.

Thanks

 Good afternoon
I want to know if someone can help me
 I'm very nervous
Pd. 5/17
Green Card in my hand 7-18-19
I just recived e-mail from Uscis that says
notice was returned to USCIS because the post office could not deliver it
the also say that can't be affect my status
I really don't know what to do

My adopted mother adopted me in Africa when I was 16 and I came to the states at 21 I am now 25
I want to obtain my citizenship but she does not want to help me what can I do?????!??

I live in Houston .. and I'm a US Citizen. We have 3 biological Kids ages (20, 18, and 16) ... we have adopted 3 kids (ages 4, 6, and 11) in May 2014 ... they were on visit visa. Their father was seriously ill and have cancer and we want to take care of the kids and provide the best we can ...... Kids are living with us since June 2013. I filled for change of status in Nov 2015 and my priority date is Nov 27.
Time Line
Kids living with me since June 2013
Court approved adoption on May 2014
Birth Certificates issued by state of TX under my name and my wifes name as parents,
File for change of status and Green Care in Nov 1015 .. send forms to Chicago office .. they transfer the docs to Benefits Center in St. Louis ..
Priority Date is Nov 27, 2015
Finger Prints are done on Dec 3, 2015
Still waiting to hear from USCIS ...

Anyone can please help to provide what will happen next ......I have no clue about what to expect next .... I'm expecting they will call me for interview but not sure when ... kids are minor and what they will be asking them?
Your help will be highly appreciated.

Thanks