Even though the IRS issues 9 out of 10 tax refunds within 21 days, it's possible your return may require additional review and take longer to process.
If it has been more than 21 days since your return was accepted, you can contact the IRS by phone at 1-800-829-1040 and they will be able to check the status for you.
Your federal refund can take longer than 21 days to process for the following reasons:
IRS representatives can only research your refund if it’s been at least 21 days since you e-filed, at least 6 weeks since you mailed your return, or if their Where’s My Refund? lookup tool tells you to contact them.
If Where's My Refund? says your return has been received but your refund has not yet been approved, that means the IRS has your return and is actively working on it.
Please see below for more details.
https://ttlc.intuit.com/questions/1901303-why-is-my-irs-refund-taking-longer-than-21-days
https://ttlc.intuit.com/questions/1901548-why-do-some-refunds-take-longer-than-others