My mother is entitled to 6 months of retroactive payment from Social Security due to the RIB-LIM rule. My father had taken Social Security 3 years early, so my mother should have taken her payment immediately after he died in December 2016. However, she thought she had to wait until her FRA, and I didn't know about RIB-LIM until I came across your website.
She went to the SSA earlier this week, and was not given the retroactive payment. Is this usually automatically given or does one have to specifically ask for the retroactive payment to receive it? What specific documentation can I show to the SSA if we go back so that she receives the payment? There is very little information about this online besides your website.
The Social Security representative who handled your mother's claim should have recognized her eligibility for retroactive entitlement and should have assigned her the appropriate month of entitlement. If that wasn't done and your mother's claim has already been processed, probably the safest route would be to have her file a formal reconsideration request (https://www.ssa.gov/forms/ssa-561.pdf). If you could reach a claims representative or technical expert at Social Security who knows what they're doing, the problem could be corrected without a formal appeal. Unfortunately, finding someone like that is hard to do these days due to agency underfunding and a lack of proper training for new employees.
It may be helpful in resolving the problem to print out and submit with your mother's appeal a copy of the following reference from Social Security's operations manual, specifically section D.2.b: https://secure.ssa.gov/apps10/poms.nsf/lnx/0200204030.