There is nothing further you can do. The missing payment may be issued at a later date since the IRS will be reviewing payments against filed 2020 returns later this year and issuing make-up checks. Or, if you don't receive it then, you'll have to wait until you file your 2021 tax return.
And even if they had not yet filed their 2020 return, I think it's the case that you can only go after missing payments that were actually approved in
2020 (i.e., the first two checks) on the 2020 return. So yes, you might have to wait a year until you file for 2021.
Two other things you can try in the meantime are:
-- Call the IRS at 800-919-9835, which is supposedly a number dedicated to the stimulus checks. You have to wade through a lot of prompts and messages, and you'll probably wait a long time on hold, but you may get lucky. More info here:
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-- Call your local Congressperson's office. Depending on who your representative is, they may have additional resources to help with individual situations like this. Again, it's a bit of a long shot, but it may be worth trying.
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