My 1099-R on line 1 (gross distribution) and line 2a (taxable amount) have same figure but Turbo tax is not indicating that any tax is due

My 1099-R on line 1 (gross distribution) and line 2a (taxable amount) have same figure and 2 b (taxable amount not determined) is checked. Some tax was paid Box 4 (federal income tax withheld) but need to pay more. However Turbo Tax is NOT calculating any tax liability. In fact it is wanting to refund the amount of tax previously paid. I looked at last year's 1099-R form and same boxes were checked and it did correctly show my tax liability. Any advice?