If you filed for the EITC (earned income tax credit) and/or the ACTC (additional child tax credit), per the IRS: "According to the Protecting Americans from Tax Hikes (PATH) Act, the IRS cannot issue EITC and ACTC refunds before February 15. The IRS expects the earliest EITC/ACTC related refunds to be available in taxpayer bank accounts or debit cards on February 27, 2019, if these taxpayers chose direct deposit and there are no other issues with their tax return."
That could be why your refund has not been released yet.
Keep checking 'where's my refund' for an exact date: https://www.irs.gov/refunds