Backdated Pip

Hi, I completed my Pip review form over a year ago, January, 2024. I received a letter 24th February, 2025, I have been awarded enhanced daily living and mobility (I previously got standard daily living). I haven't had my payment backdated? I rang Pip, and the person who made the decision said they could backdate if I had a new condition and then they need evidence when to backdate it, letters from hospital etc. I told him I needed a new knee replacement and I think I can find a letter. I also said to get the higher award then you must already know there's a change and why should I be penalised because it took over a year for my review to be looked at? He just said he is going to backdate it but until I send further evidence, he doesn't know when to backdate it to. I found the hospital letter dated May, 2024 but shouldn't I have my Pip backdated to when I got the review form, in January, 2024?