If the IRS "Where's My Refund" tool says your return is still being processed, this IRS FAQ gives some situations where it might take longer than 21 days:
https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
This time of year one of the more common reasons for a delay is the PATH Act which affects filers who had EITC or ACTC tax credits in their return. By law the IRS cannot release those refunds until after February 15. So some filers who had those credits may not see a refund until around March 1, although if one chose direct deposit, they may get it earlier than that, such as later in February.
If the PATH Act (i.e., one of those EITC/ACTC credits) is the reason for the delay, the IRS says it hopes to update the "Where's My Refund" tool "by February 19" to show the expected deposit date for most filers in that situation.
