child credit less than last year

last year (2019 tax year) i got the 2k per child/dependent tax credit and since i have 2 kids, it was a total of 4k.  this year (2020 tax filing year) i'm only getting a total of 2.5k child/dependent tax credit... We're filing married jointly, one child is 12 years old, the other turned 17 on december 2020. We made less income in 2020 than in 2019... none of the kids worked, they're both in school fulltime.  We are not filling a complicated income tax return, just your regular family tax return.  Can someone give me an i dean why the tax credit is so low this year?