SDP Disregard

Hello i should have been given sdp September of 2022 when my daughter became ill and awarded pip and lcwra UC. I was told February this year i was entitled to it and wrote to dwp asking took 4 letters they eventually awarded me but refused to backdate my claim. Please can anyone advise how word myself to ask why? Thank you so much.

I have asked and they’re saying it was my responsibility to have let them know. I have since been told of a caselaw that it wasn’t my responsibility but theirs. I have seen a response from Gary to someone in similar situation that he said he helps with sdp and backdating. Thank you