We do not know why --- but the first payments only went out today. There are bound to be bugs and glitches in the system at first. Congress changed the tax laws in the middle of the tax season and the IRS had to throw together a way to send out millions of payments ---- while already backlogged with tax refunds and stimulus payments.
If you do not get it as monthly payments you will be able to get it on your 2021 tax return. Give it some time--see if they work it out soon.
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-f...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
