Are you saying you have dependents under the age of 17 that you did not receive a $500 stimulus payment for? Several people have posted that this has happened. Unless the IRS comes up with a way for you to contact them about this, you will have to wait until you filed your 2020 tax return early next year to get the missing amounts.
The stimulus payment is actually an advance on your 2020 tax return. If you should have received more payment than you got and you still qualify for the additional amounts on your 2020 tax return, you will get additional credits on that return for the missing amount.