Processing of returns is not a simple task ... the "pipeline" has lots of stops along the way. Some returns will slide straight thru quickly and some returns will be stopped for review at some point and maybe more than once. Each "review" may take a while depending on what is being checked and how busy that checkpoint is. There is nothing you can do about it but wait patiently. Check the IRS site every couple of days to see when the return is finally approved and the refund is distributed.
***If your return has anything on lines 27 and/or 28 then your return will not start processing until mid Feb due to the PATH Act put in place several years ago and don't expect your refund any earlier than 3/1.
https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions
