If you only put your $1,800 (1200+600) in Turbo Tax then it would give you the $1,800 (1200+600) for your spouse. You need to enter the $3,600 total for both of you. Check your bank accounts again. The IRS thinks they sent you $3,600 (1,800 each). If you only entered the 1200 deposit you think you got, that's why line 30 had 2400 on it for the missing amount. You don't get the first 2 amounts for child if they were over 17 on Dec 31, 2020.
If you can't find deposits, checks or debit cards for the missing $2,400 they will need to put a trace on it.
Your dependent should not have received any Stimulus payments in their own name. The $1,400 3rd one should have come to you in your ssn. Did they file their own return and say they were NOT a dependent?
How to put a trace on a missing 1st or 2nd Stimulus payment. IRS for Payment Issued but Lost, Stolen, Destroyed or Not Received
See question F3 here, you can expand it to see how to put a trace on it.
https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-f-finding-the-first-and-second-econom...
