I didn’t receive all my child credits. None of the reasons listed in the letter from the IRS apply to me. How should I get it corrected?

I double checked all four of my kids’ information on my return and it’s all correct and it is the same from last year. Also, our combined income was below the threshold. I’ve been calling the IRS once a week since March and can’t get through. My local IRS office doesn’t even answer the phone. Should I resend my return with copies of my kids’ birth certificates?