After you file

The 21 days is just a time frame in general, based on averages.  There are any of a number of reasons it can take longer, however.   

When you enter your info into the IRS "Where's My Refund" tool <a rel="nofollow" target="_blank" href="https://www.irs.gov/Refunds">https://www.irs.gov/Refunds</a> , what does it show?  If it says your return is still "being processed", here's how to phone the IRS to check on it:

IRS:  800-829-1040  hours 7 AM - 7 PM local time Mon-Fri

Listen to each menu before making the selection.
 
First choose your language. Press 1 for English.

    Then do NOT choose the first choice re: "Refund", or it will send you to an automated phone line.
    Instead, press 2 for "personal income tax".
    Then press 1 for "form, tax history, or payment".  
    Then press 3 "for all other questions."
    Then press 2 "for all other questions."  It should then transfer you to an agent, but there is likely to be a long wait.

I usually use a speakerphone so I can work on something else while waiting.