Still having trouble regarding 3rd Stimulus payment

I checked my bank and my canceled checks and do not see a deposit for the amount IRS claims to have sent.  I did get Child payments directly to my bank so those were there.  I called IRS and they claim a check was mailed in March 2021 for $5600 even though the IRS letter I got was for $2,800. So I guess my husband should get a letter for the other $2,800.  Anyway, they told me on line 30 to fill in $5600 and claim recovery rebate. But If I did not get the money how does claiming it help?