I found that even though the *paper* version of my W-2 I received did not have Box 13 (Retirement plan) checked, the electronic version TurboTax imported from ADP did have it checked. I did find a place to uncheck the option in TurboTax which though I "was covered by employer retirement plan". Unfortunately, as soon as I saved/exited/restarted TurboTax, the software again silently lost my IRA contribution deduction.
To fix, I went directly to the "forms", selected my W-2, unchecked Box 13, saved/exited/restarted, and this time my IRA contribution was being honored correctly in TurboTax.
