Yes, in fact there are several reasons. First, in HealthCare.gov states, when you first applied for coverage, you had the opportunity to authorize the Marketplace to check online income data about you, including from your tax returns, for another 1 to 5 years. If you did not authorize this (note that most people did give authorization), your financial assistance will NOT be automatically continued for next year. You must re-apply for financial assistance if you want the APTC to continue for another year.
Second, if you did authorize the Marketplace to check income information about you, it will check the most recently available information, which for most people will be your 2019 federal income tax return. If the income you reported on that tax return was more than 500% of the federal poverty level (roughly $64,000 for a single person, $131,000 for a family of 4), your financial assistance will not automatically be continued for 2021. Instead, to continue receiving APTC in 2021, you will have to re-apply for financial assistance and provide information about your expected 2021 income.
Third, if you received a premium tax credit in 2019 but then, this year, you did not file a 2019 federal income tax return including Form 8962, your financial assistance will NOT be continued for 2021. You will need to file a 2019 tax return as soon as possible, including a completed IRS Form 8962. Once you have filed, if you live in a HealthCare.gov state, log into your account, update your application, and be sure to check the box telling the Marketplace that you filed a 2019 tax return and reconciled your premium tax credits.
Keep in mind that later during the coverage year the Marketplace will check with the IRS to verify your return was filed and, if it cannot verify, will terminate your premium tax credit.
Note that the process for renewing financial assistance may vary in other states. All state Marketplaces will send consumers notices prior to the start of Open Enrollment explaining the process for continuing or re-applying for financial assistance for the coming year.