LCWRA back pay

I have in may 2022 been awarded LCWRA. I have handed in fit notes continuously since 2020 and had a payment every month on the 11th since then without miss. 
My first fit note handed in on the system is in December 2020. I believe that I made universal credit aware of my health condition before this and due to national lockdown I didn’t require a health note as no appointments was necessary. 
I have never been off universal credit since then, I have only changed circumstances like gone from a couple to single etc but always received universal credit payments and every month  have given in fit notes when needed. Therefore my claim has never closed only adjusted. 
I have been entitled to extra pay in all these payments since 2020 and haven’t received it, therefore I’m entitled to the back pay since it says in the awards letter ‘the first time I handed in a fit note’. Given this situation this would have been summer 2020 but as mentioned previously because of lockdown the first couple of fit notes weren’t needed which I was told by univerisal credit and the first fit note provided was in December 2020. 
This has been confirmed by my local job centre in Littlehampton and on the universal  credit helpline. 
My case manager somehow thinks that it doesn’t apply to me because I’ve had a claim closed. Which I haven’t, I’ve had universal credit payments every month on the 11th since 2020. The only thing that’s changed in my claim is my circumstances but nevertheless I’ve still provided fit notes every month since 2020 and received payments every month 11th which you can check on your system. 

I believe I’m entitled to the back pay from summer 2020 or very least December 2020 when the actual first fit note was given. My claims never been closed so that’s ridiculous to be told that, especially given all these days I should have been given the extra health pay I’ve been entitled to.