Deductions & credits

when I go the IRS website it states the qualifications. I fall under the qualifications. So I filed 3 children on my 2020 tax return. I recieved advancements for 3 children. I am suppose to recieve the other half for my 2 children on my 2021 return, which would be 3000 instead I'm recieving 1500, which means they are taking back the 1500 I recieved for the child I'm not claiming this year. How am I not paying it back? I am.