The Department of Treasury's Financial Management Service (FMS) can offset any unpaid federal or government debt against your tax refund.
Most commonly, the offset is taken for:
For further assistance call the FMS at 1-800-304-3107 to find out if your refund was reduced due to an offset.
You may want to request a "Record of Account Transcript" from IRS to see what happened. The IRS may have changed an amount on your return during processing, but for some reason you didn’t receive a notice.
Record of Account Transcript - combines the tax return and tax account transcripts into one complete transcript. This transcript is available for the current tax year and returns processed during the prior three years using Get Transcript Online or Form 4506-T at: https://www.irs.gov/individuals/get-transcript