Taxable amount of IRA Distribution

I received a 1099-R showing a Gross Distribution of $17,000, with the Taxable Amount not determined box checked.  $3,000 was a loan I repaid, so $14,000 is taxable, which I entered in Box 2a Taxable Amount in TurboTax, but it keeps picking up the full $17,000 in its calculations.  Any suggestions?