There's no way for us to know.  Your return might have  problem, or it might be a random extra check.  If it is more than 3 weeks you can call the general IRS number and ask for an update.  1-800-829-1040

If there is a serious issue they should mail a letter to the address on your return.  If it is more than 8 weeks you can try calling the office of taxpayer advocate. https://www.irs.gov/Advocate