Back date DLA?

Hi. My son had a review of his DLA. We were approaching the end of the entitlement date so they sent a new form. It took over 4 mths for a decision but he has gone from middle to high rate care. They have issued this increase from the date he was next due his payment. The letter is dated 9.02.22 and his award will increase 23.2.22. So we have just got a payment of the older, lower amount and now I presume it will be 4 wks before the next payment of the increased amount? Does this sound correct? Do they no longer back date to when the form was completed?? Any advice gratefully received

OK, thanks for advise.