PIP mixing with people

Hi. I've recently had my PIP re-assessed and in the information I mentioned the how I only scored 2 points previously for mixing with people when I rely on support for this. They have now agreed that I need support to mix with people but have not backdated this further than one month. I read your emails and believe I should fall in the catagory of people who are owed a back payment for getting this wrong when I first applied for PIP. Nothing was mentioned about this wrong decision despite me pointing it out to them when they looked again at my claim. Do I have to wait for them to get round to it in their own time or can I prompt them to look at this mistake now? I would be very grateful for your advice with this matter??