The IRS is hopelessly backlogged, with 35 million unprocessed tax returns and 2 new laws to follow (stimulus round 3 and the advance child tax credit payments). It's a miracle that even some people have been paid.
Remember the advance payments don't change the amount you get, only when you get paid. If the credit starts late and you don't get the full amount, it will be added to your tax refund.
