What to do when it's been 21 days and I haven't received my federal refund?

It's been 21 days and I haven't received my federal refund. I checked the, "Where is my refund" on the irs.gov. I keep getting the status say, "Your refund have been received but is being processed". I don't understand what's wrong because I efiled my taxes. I need some help on what to do.