They are supposed to mail you a letter 15 days after they issue your payment, providing a number for you to be able to call for issues regarding your stimulus. But know that it will likely be incredibly hard to get someone on the phone for the IRS for the next several weeks as they are understaffed and completely swamped with calls. Still, worth trying.
It won't help you right now, but you can still get the $500 once you file your 2020 taxes. This stimulus is an advance on a 2020 tax credit technically, so if you were shorted, you will be able to get it when you file next year. Again, I know that doesn't help now, but worst case scenario the money will come to you. Hopefully you can get through to the IRS through the number they provide in relative short order.