Issues with National Insurance Number

I'm honestly just looking for guidance because I'm really confused with what to do.
I keep getting the same letters from SLC asking me to confirm my surname so they can verify my national insurance number. Apparently the DWP can't verify my surname? I have two surnames, which I'm thinking is the issue. They may think that the first half of my surname is my middle name or something. On every application I've ever done, I've put both my surnames in the surname box, and in every personal document I have, the surnames are correct and in the surname section. I've already sent them documentation of my national insurance number and It's been returned to me. Now they've sent me the same letter? Do they want to see my passport? How do I show them that I have a two name surname? I've tried to call the number they provided, but it was just general, automated responses.

Any advice will help me, thanks

Hi am trying to send my NI number and name , do they want my maiden name as they aren't seeing my married name , can you help me please.