https://www.mercurynews.com/2021/04/05/who-will-get-plus-up-stimulus-payments-from-the-irs/

(CNN) — The Internal Revenue Service started sending what it calls “plus-up” payments last week to people who are owed a bigger stimulus check than they initially received.

The original payments were based on 2018 or 2019 tax returns. That means people who earned less in 2020 could be owed more money. Once they file a 2020 tax return that shows a lower income amount, the IRS will automatically send them an additional payment.