I suspect the IRS will rigidly adhere to what their computer says you received, and will reduce your refund by the amount they think they paid, even if you didn't receive it.
Are you missing specific payments, like a particular month? Did you change bank accounts or change between bank account, check or debit card?
You may need to look at the procedures for tracing missing payments, rather than trying to claim the missing payment on your return. See here for more.
https://www.taxpayeradvocate.irs.gov/news/tas-tax-tip-did-you-have-an-advance-payment-of-the-child-t...
