The 21-day period is an estimate based on past IRS experience with efiled returns that chose direct deposit for the refund. While many returns finish processing by then, many do not. If the status doesn't change in the next 2-3 weeks, you could phone the IRS, if you wish to see if they can tell you anything about the delay. Also keep an eye on the IRS "Where's My Refund" tool in case they post any message there. Occasionally they will ask for more info or will send a letter asking for such. Some returns are subjected to extra processing on a random basis. I assume you've been checking on the progress with this tool:
IRS "Where's My Refund" tool:
https://www.irs.gov/wheres-my-refund