Your federal refund can take longer than 21 days to process for numerous reasons. https://ttlc.intuit.com/replies/3302330

You'll get an explanatory letter from the Bureau of the Fiscal Service if your refund is offset. The remainder of your refund will be processed as usual; an offset shouldn't delay it, unless of course your entire refund was applied to your debt. https://ttlc.intuit.com/replies/4143297