There are different types of review processes that the IRS might place you under.
The IRS might place you on an automatic "60-Day review", with this review the IRS usually will send you a 4464C letter advising you of the delay.
Understanding your letter 4464C
You might also see a delay due to Tax Topic 152 references that your return might need additional review and could take more than the usual 21 days. This doesn't mean that there's anything wrong with your return, or that there's anything missing. For more information regarding this topic, visit the link below
Tax Topic 152: Will I Still Get My Tax Refund?
There could also be a review for Identity verification. The IRS will also send you a letter for this to verify your identity using instructions located on that letter.
If your review process doesn't fall into any of these categories, contact the IRS directly for further information by clicking the link below
IRS HELP