After you file

• You claimed an Earned Income Tax Credit and/or an Additional Child Tax Credit and filed before February 15, 2018.  It takes until the week of February 27 to receive your refund at the earliest.
• The IRS processed a refund offset.
• There were errors, inconsistencies, or missing info on the return.
• The return is under review.
• The IRS suspects identity theft or fraud.
• There's a Form 8397 (Injured Spouse Allocation) on your return.  It takes 11-14 weeks to process.
• You filed Form 1040-X (Amended Return), which can take 12-16 weeks to process.

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

https://ttlc.intuit.com/questions/1894389-when-will-i-get-my-irs-tax-refund