Paul Johnston

I done my first self-assessment last year for the 2020-21 tax year. I paid in full the total amount owned for 2020-21 but I also paid one advance payment for the 2021-22 tax year. A few weeks ago I worked out and declared what I owe for the 2021-22 tax year. Its been now processed by the HMRC and they sent back the amount that I owe. The total amount owed is the amount that I declared. What I don't understand why the advanced payment I made for 2021-22 hasn't been deducted off the amount that I owe. Should I've included the advance payment myself and declared less? Can anyone tell me why the advanced payment hasn't been taken off the amount due for 2021-22?