Did you enter the Social Security Form SSA-1099 correctly in TurboTax? The federal taxes withheld are entered on the same screen as the benefits received.
To enter Social Security benefits reported on form SSA-1099
Or enter ssa-1099 in the Search box located in the upper right of the program screen. Click on Jump to ssa-1099
Up to 85% of Social Security Retirement/Disability/Survivors benefits becomes taxable when all your other income plus 1/2 your social security reaches: