Why do you believe that the taxable amount is less than the full amount?
A Form 1099-R reporting a regular distribution from a traditional IRA must have box 2a equal to box 1 (except for a distribution from an inherited IRA indicated with code 4 in box 7 where TurboTax requires you to enter a different taxable amount if the decedent had basis in nondeductible traditional IRA contributions).
If you have basis in nondeductible traditional IRA contributions, you must indicate this and the total year end value in all of your traditional IRAs after clicking the Continue button on the page that lists the Forms 1099-R that you have entered. TurboTax will then prepare Form 8606 to calculate the taxable amount of your traditional IRA distribution.
