I would contact the embassy to see if they can update you. They will most likely have you contact them via email (that's what happened to me).
Side note: What documents did your mother miss at the time of the interview?
Honestly, at this point, this feels like an IRS issue. Your mother may have to contact the IRS to see what she needs to do.
Whether that will open a can of worms or not is unknown. Good luck!
Which category does your mom fall under in the IRS threshold for filing:
https://www.irs.gov/newsroom/who-needs-to-file-a-tax-return
if she married and filing separately or if she is married filing together and reached the threshold than you have another issue on your hand.