Why did I get the additional child tax credit this year and not last year?

[KrisD15]

The Additional Child tax Credit is an extension of the Child Tax Credit.

When the Child Tax Credit is applied, it lowers your tax liability.

If your tax is lowered to zero, but the amount of the child tax credit was not totally used up, the remaining (up to 1,600) is applied and called the "Additional Child Tax Credit" which now can increase your refund. 

Last year the Child Tax Credit must have been applied in full, so the additional child tax credit was not required to use the full amount as a credit. 

[Hal_Al]

The additional child tax credit is a calculated amount. Sometimes, the  calculated amount is $0. Look at your tax forms for both years, particularly form 8812.

The child tax credit  (CTC) is limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $2500 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 28 of form 1040 (2021- 2023) instead of line 19. The  ACTC is calculated on form 8812 and  is basically 15% of your earned income over $2500. The ACTC is a maximum of $1600 (2023) ($1500 for 2022) per child, not $2000.