How can I get Turbo Tax to use Box 2a Taxable Amount instead of Box 1 the gross amount in my 1099R (G)?

How can I get Turbo Tax to use Box 2a "taxable amount" of my annuity instead of Box 1 "gross amount" from my 1099R (G)?  Last year (and years before that) Turbo Tax used Box 2a, but not this year.  Thanks for your help!