IRS website said the tax return is being reviewed and a letter would be sent. I haven't received a letter but a small amount was deposited not the rest of my refund.

If you owe certain delinquent debts, such as student loan debt, that were submitted for offset, your refund can be used to pay off these debts. You can call the Treasury Offset Program Call Center at 1-800-304-3107 at any point to learn more about the amount of your debt.  The IRS or the Department of Treasury's Financial Management Service (FMS), which issues IRS tax refunds, can offset or reduce your federal tax refund or withhold the entire amount to satisfy the debt. Any portion of your refund remaining after an offset will be refunded to you.