For the stimulus check only $600 have arrived. Our gross income is below the $150K (we filed together). I want to know where are the other $600. Thanks.

Have you gone to the irs.gov website to check on your status by entering "get my payment" in the search box?

IRS has set up a process to help you enter identifying information and check the status of your second payment.  It will also confirm the payment of your first check.  Good luck - haven't got mine either.

Go to this IRS get my payment website - https://www.irs.gov/coronavirus/get-my-payment

If you do not receive any other stimulus payments by January 15, 2021, then the stimulus payment for which you are eligible to receive will be entered on your 2020 federal tax return as a tax credit on Form 1040 Line 30.

The TurboTax program will ask about stimulus payments in the Federal Review section of the program.