Updated my son to have SSN from ITIN but return rejected as it says IRS didn't recognize SSN. I just received card ….any ideas why rejected....

I have gotten a new SSN for my son so I can get the tax credit but the IRS rejected my return.  I know they check against the social security so not sure why they are not in synch.  Does anyone know if social security office has different system to work with IRS... any ideas why this didn't work?